Oil well pipe connection structure and oil well pipe

ABSTRACT

Provided is means for connecting oil well pipes together with a compact/simple structure and the strength that significantly exceeds the limits of prior art even without the flange, etc. An oil well pipe coupling structure includes a first oil well pipe and a second oil well pipe, the external thread portion has a first external thread portion, a substantially tapered second external thread portion, and a third external thread, the internal thread portion has a first internal thread portion, a substantially tapered second internal thread portion, and a third internal thread portion the first external thread portion is screwed to the first internal thread portion, the second external thread portion is screwed to the second internal thread portion, the third external thread portion is screwed to the third internal thread portion, and the first oil well pipe and the second oil well pipe are coupled together.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit under 35 USC §119(a) of Japan Patent Application No. 2020-056905 (filed on Mar. 26, 2020), Japan Patent Application No. 2020-128988 (filed on Jul. 30, 2020), Japan Patent Application No. 2020-137153 (filed on Aug. 14, 2020), Japan Patent Application No. 2020-214705 (filed on Dec. 24, 2020), Japan Patent Application No. 2020-215878 (filed on Dec. 24, 2020), Japan Patent Application No. 2021-003833 (filed on Jan. 13, 2021), Japan Patent Application No. 2021-027058 (filed on Feb. 24, 2021), Japan Patent Application No. 2021-040863 (filed on Mar. 12, 2021), and Japan Patent Application No. 2021-040867 (filed on Mar. 12, 2021), in the Japan Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes.

BACKGROUND 1. Field of the Invention

The present disclosure relates to an oil well pipe coupling structure and an oil well pipe.

2. Description of the Related Art

Use of a plurality of hollow tubular members such as oil well pipes coupled together in an axial direction is commonly known, and, as coupling means, coupling two members by joining through welding or the like and coupling two members with a separate member (a sleeve, a threaded pipe joint, or the like) are known.

As the separate member, for example, in the coupling of pipes as threaded tubular members, an integral-type threaded pipe joint that directly connects the pipes with a pin (external thread side) and a box (internal thread side) is known (for example, refer to Patent Document 1 (Japanese Patent No. 6037091)). In such a threaded pipe joint, an intermediate shoulder is provided in the middle of the thread line of a tapered thread, and the thread gap is set to be wider in the thread line on the inner side than in the thread line on the opening side of the intermediate shoulder in the axial direction.

In addition, in the case of connecting oil well pipes each having a tapered external thread in the end portion via a threaded joint (for example, refer to Patent Document 2 (Japanese Publication No. 2017-072187)), the oil well pipes are screwed into the internal thread portions of the threaded joint on both ends, and thereby the thickness portions of the oil well pipes that have become thin are surrounded by the threaded joint, which improves the expansion and contraction resistance, the bending resistance, the external pressure and internal pressure resistance, and the like of the oil well pipes.

In addition, a structure in which one steel pipe provided with an external joining end portion having thread on the outer circumference and the other steel pipe provided with an internal joining end portion that has thread on the inner circumference and is to be fitted with the external joining end portion are joined together by fitting the joining ends together is known (refer to Patent Document 3 (Japanese Publication No. 2019-163781)).

In addition, it is known that steel pipes are joined together by providing a flange to an end portion of each steel pipe, putting the flanges together, passing a bolt through the flange holes, and fastening the bolt with a nut (refer to Patent Document 4 (Japanese Publication No. 2017-040074)). For such a steel pipe with a flange, ordinarily, the shape of the flange, the number of bolts, and the like are determined based on the designed pressure of the steel pipe. That is, the steel pipe is designed to secure a strength high enough to withstand an internal pressure that is exerted at all times and to cause a sufficient surface pressure to act on the flange joint surfaces so as to prevent the occurrence of leakage.

However, in the case of coupling members together by welding, there is an issue in that a long period of time is taken and workability deteriorates.

In addition, when the external thread or the internal thread formed on a pipe is a straight thread as in Patent Document 3, there is an issue in that the obtainable maximum strength of the pipe in the axial direction is just approximately less than 50% of the total cross-sectional strength. The strength of the pipe in the axial direction can be improved by turning the straight thread into a tapered thread; however, even in such a case, the obtainable strength of the pipe in the axial direction is just approximately 70% of the total cross-sectional strength in actual situations.

When pipes are connected together via the threaded joint of Patent Document 2, it is possible to obtain a strength as high as the total cross-sectional strength, but the threaded joint has a larger diameter than the pipes, which makes a place where the threaded joint is disposed larger. Therefore, at the time of disposing pipes connected together via the threaded joint in the ground, time and effort are taken to increase the size of a hole or the like. In addition, crude oil or natural gas wells are becoming deeper, horizontal wells or inclined wells are becoming more widespread, as opposed to vertical wells, and development of an increasing number of wells under harsh environments such as the ocean or polar regions is underway. In such well development environments, an increase in the sizes of holes creates an issue of an increase in working costs.

In a case where a fluid passes through pipes, when the threaded joint of Patent Document 1 or 2 is used or when pipes are directly screwed and connected together as in Patent Document 3, a seal property that prevents the leakage of the fluid is secured in the pipes by bringing the surfaces of the pipes into contact with each other or by bringing the surface of the threaded joint into contact with the surfaces of both pipes. In addition, even in a case where pipes are joined together by welding, a seal property is secured by joining. However, when a pipe or a joint expands at high temperatures or receives a large load and thus deformation (bending, elongation, or the like) occurs or a crack is generated in a joint portion, there is an issue in that a gap is formed between the pipe and another pipe or between the pipe and the joint, which negates the seal property.

In addition, in the case of joining the steel pipes with a flange of Patent Document 4, it is not unusual that, in addition to an internal pressure, an axial force or an external force such as bending is exerted. Even for flanges having a strength high enough to withstand internal pressures, in environments under external forces, there is an issue in that the flange deforms due to the exertion of a moment, a surface pressure acts on the flange joint surface, and leakage occurs. In addition, there is another issue in that it is necessary to increase the size of such a flange in order to prevent the deformation of the flange. Furthermore, there is still another issue in that a large storage place is required at the time of storing the steel pipes with a flange in an arrayed state.

SUMMARY

The present disclosure has been made by the present inventors' intensive studies in consideration of the above-described problems, and an object of the present disclosure is to provide means for connecting oil well pipes together with a simple structure in which the sizes of the oil well pipes are not increased, but are confined within the inner and outer diameters of the oil well pipes and the oil well pipes have a strength significantly surpassing the limit of the related art.

In addition, another object of the present disclosure is to provide means for forming a sealing structure for oil well pipes capable of maintaining a high sealing performance at high temperatures or under environments in which the oil well pipes receive large loads or the like.

An oil well pipe coupling structure of the present disclosure includes a first oil well pipe having an external thread portion on an outer circumferential surface and a second oil well pipe that has an internal thread portion that is screwed to the external thread portion on an inner circumferential surface and can be connected to the first oil well pipe, the external thread portion is made up of a first external thread portion having a diameter of a predetermined dimension and a substantially tapered second external thread portion that is disposed on one end portion side with respect to the first external thread portion and gradually contracts radially toward one end side, the internal thread portion is made up of a first internal thread portion that is disposed on an opening end side and has a diameter of a predetermined dimension and a substantially tapered second internal thread portion that gradually contracts radially from the first internal thread portion side toward an inner side, the first external thread portion is screwed to the first internal thread portion, the second external thread portion is screwed to the second internal thread portion, and the first oil well pipe and the second oil well pipe are coupled together.

In addition, in the oil well pipe coupling structure of the present disclosure, a diameter of the second external thread portion is equal to or smaller than the diameter of the first external thread portion, and a diameter of the second internal thread portion is equal to or larger than the diameter of the first internal thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, a thread shape in the first external thread portion and/or the second external thread portion forms a substantially saw blade shape, and a thread shape in the first internal thread portion and/or the second internal thread portion forms a substantially saw blade shape.

In addition, in the oil well pipe coupling structure of the present disclosure, in the thread shape of the first external thread portion and/or the second external thread portion, a flank angle of a flank surface that receives a pressure when the first oil well pipe and the second oil well pipe are pulled to be separated from each other along an axial direction from a coupled state of the first oil well pipe and the second oil well pipe is equal to or smaller than a right angle with respect to an axial core of the first oil well pipe.

In addition, in the oil well pipe coupling structure of the present disclosure, in the thread shape of the first internal thread portion and/or the second internal thread portion, a flank angle of a flank surface that receives a pressure when the first oil well pipe and the second oil well pipe are pulled to be separated from each other along an axial direction from a coupled state of the first oil well pipe and the second oil well pipe is equal to or smaller than a right angle with respect to an axial core of the second oil well pipe.

In addition, in the oil well pipe coupling structure of the present disclosure, the first external thread portion and the second external thread portion are equal to each other in pitches of thread.

In addition, in the oil well pipe coupling structure of the present disclosure, the first internal thread portion and the second internal thread portion are equal to each other in pitches of thread.

In addition, in the oil well pipe coupling structure of the present disclosure, pitches of thread in the external thread portion and pitches of thread in the internal thread portion are equal to each other.

In addition, in the oil well pipe coupling structure of the present disclosure, in the external thread portion, pitches of thread gradually decrease from the first external thread portion side toward the second external thread portion side, and, in the internal thread portion, pitches of thread gradually decrease from the first internal thread portion side toward the second internal thread portion side.

In addition, in the oil well pipe coupling structure of the present disclosure, an outer diameter of a non-engagement region in the first oil well pipe with respect to the second oil well pipe is substantially equal to an outer diameter of the second oil well pipe.

In addition, in the oil well pipe coupling structure of the present disclosure, an inner diameter of a hollow in the first oil well pipe is substantially equal to an inner diameter of a non-engagement region of the second oil well pipe with respect to the first oil well pipe.

In addition, in the oil well pipe coupling structure of the present disclosure, a region along an axial direction is longer in the second external thread portion than in the first external thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, a region along an axial direction is longer in the second internal thread portion than in the first internal thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, the external thread portion has a third external thread portion that is disposed on the one end side with respect to the second external thread portion and has a diameter of a predetermined dimension, the internal thread portion has a third internal thread portion that is disposed on the inner side with respect to the second internal thread portion and has a diameter of a predetermined dimension, and the third external thread portion can be screwed to the third internal thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, the third external thread portion has an effective diameter that is equal to or larger than an effective diameter of the second external thread portion and has a straight thread shape, and the third internal thread portion has an effective diameter that is equal to or larger than an effective diameter of the second internal thread portion and has a straight thread shape.

In addition, in the oil well pipe coupling structure of the present disclosure, a region along an axial direction is longer in the second external thread portion than in the first external thread portion and/or the third external thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, a region along an axial direction is shorter in the second external thread portion than in the first external thread portion and/or the third external thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, a region along an axial direction is longer in the second internal thread portion than in the first internal thread portion and/or the third internal thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, a region along an axial direction is shorter in the second internal thread portion than in the first internal thread portion and/or the third internal thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, the external thread portion has a first circular region including the first external thread portion, a second circular region including the second external thread portion, and a third circular region including the third external thread portion that are virtually divided in a radial direction, an area of the first circular region is equal to or smaller than a third of a total cross-sectional area of a transverse section of the first oil well pipe, an area of the second circular region is equal to or larger than a third of the total cross-sectional area of the transverse section of the first oil well pipe, an area of the third circular region is equal to or smaller than a third of the total cross-sectional area of the transverse section of the first oil well pipe, the internal thread portion has a fourth circular region including the first internal thread portion, a fifth circular region including the second internal thread portion, and a sixth circular region including the third internal thread portion that are virtually divided in a radial direction, an area of the fourth circular region is equal to or smaller than a third of a total cross-sectional area of a transverse section of the second oil well pipe, an area of the fifth circular region is equal to or larger than a third of the total cross-sectional area of the transverse section of the second oil well pipe, and an area of the sixth circular region is equal to or smaller than a third of the total cross-sectional area of the transverse section of the second oil well pipe.

In addition, in the oil well pipe coupling structure of the present disclosure, the first external thread portion and/or the third external thread portion have a thread with a symmetrical shape, the first internal thread portion has a thread shape corresponding to the thread of the first external thread portion, and the third internal thread portion has a thread shape corresponding to the thread of the third external thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, a sealing structure is provided between an outer circumferential surface of a tip portion of the first oil well pipe and an inner circumferential surface on a base end side of the internal thread portion of the second oil well pipe and/or between an outer circumferential surface on a base end side of the external thread portion of the first oil well pipe and an inner circumferential surface of the tip portion of the second oil well pipe.

In addition, in the oil well pipe coupling structure of the present disclosure, a sealing structure is provided between an outer circumferential surface of a tip portion of the first oil well pipe and an inner circumferential surface on an inner side of the internal thread portion of the second oil well pipe and/or between an outer circumferential surfaceo a base end side of the external thread portion of the first oil well pipe and an inner circumferential surface of the opening portion of the second oil well pipe, and, in the sealing structure, a length in an axial direction of the outer circumferential surface of a tip portion of the first oil well pipe is set to shorter than a length in the axial direction of the third internal thread portion of the second oil well pipe and/or a length in the axial direction of the outer circumferential surface on the base end side of the external thread portion of the first oil well pipe is set to shorter than a length in the axial direction of the inner circumferential surface on the opening side of the second oil well pipe.

In addition, an oil well pipe of the present disclosure has a hollow structure and has an external thread portion on an outer circumferential surface, the external thread portion is made up of a first external thread portion having a diameter of a predetermined dimension and a substantially tapered second external thread portion that is disposed on one end portion side with respect to the first external thread portion and gradually contracts radially toward one end side, and the external thread portion is screwed to a different member having an internal thread portion on the inner circumferential surface.

In addition, in the oil well pipe of the present disclosure, a diameter of the second external thread portion is equal to or smaller than the diameter of the first external thread portion.

In addition, in the oil well pipe of the present disclosure, a thread shape in the first external thread portion and/or in the second external thread portion forms a substantially saw blade shape.

In addition, in the oil well pipe of the present disclosure, in the thread shape of the first external thread portion and/or the second external thread portion, a flank angle of a flank surface that receives a pressure when the oil well pipe is pulled to be separated in an axial direction from a state where the oil well pipe has been screwed to the different member is equal to or larger than a right angle with respect to an axial core of the oil well pipe.

In addition, in the oil well pipe of the present disclosure, a region along an axial direction is longer in the second external thread portion than in the first external thread portion.

In addition, in the oil well pipe of the present disclosure, the external thread portion has a third external thread portion that is disposed on the one end side with respect to the second external thread portion and has a diameter of a predetermined dimension.

In addition, in the oil well pipe of the present disclosure, the third external thread portion has an effective diameter that is equal to or larger than an effective diameter of the second external thread portion and has a straight thread shape.

In addition, in the oil well pipe of the present disclosure, a region along an axial direction is longer in the second external thread portion than in the first external thread portion and/or the third external thread portion.

In addition, in the oil well pipe of the present disclosure, a region along an axial direction is shorter in the second external thread portion than in the first external thread portion and/or the third external thread portion.

In addition, in the oil well pipe of the present disclosure, the external thread portion has a first circular region including the first external thread portion, a second circular region including the second external thread portion, and a third circular region including the third external thread portion that are virtually divided in a radial direction, an area of the first circular region is equal to or smaller than a third of a total cross-sectional area of a transverse section of the oil well pipe, an area of the second circular region is equal to or larger than a third of the total cross-sectional area of the transverse section of the oil well pipe, and an area of the third circular region is equal to or smaller than a third of the total cross-sectional area of the transverse section of the oil well pipe.

In addition, in the oil well pipe of the present disclosure, the first external thread portion and the third external thread portion have a thread having a symmetrical shape.

In addition, in the oil well pipe of the present disclosure, on a tip side and/or a base end side of the external thread portion, an outer circumferential surface that can be brought into close contact with an inner circumferential surface of the different member substantially throughout an entire circumference when the external thread portion has been fitted into the internal thread portion of the different member is provided.

In addition, in the oil well pipe of the present disclosure, on a tip side and/or a base end side of the external thread portion, an outer circumferential surface that can be brought into close contact with an inner circumferential surface of the different member substantially throughout an entire circumference when the external thread portion has been fitted into the internal thread portion of the different member is provided, and a length in an axial direction of the outer circumferential surface on the tip side of the external thread portion is set to shorter than a length in the axial direction of the third external thread portion and/or a length in the axial direction of the outer circumferential surface on the base end side of the external thread portion is set to shorter than a length in the axial direction of the first external thread portion.

In addition, an oil well pipe of the present disclosure has a hollow structure and has an internal thread portion on an inner circumferential surface, in which the internal thread portion is made up of a first internal thread portion having a diameter of a predetermined dimension on an opening end side and a substantially tapered second internal thread portion that gradually expands radially along a direction away from the first internal thread portion, and the internal thread portion is screwed to a different member having an external thread portion.

In addition, in the oil well pipe of the present disclosure, a diameter of the second internal thread portion is equal to or larger than the diameter of the first internal thread portion.

In addition, in the oil well pipe of the present disclosure, the thread shape in the first internal thread portion and/or the second internal thread portion forms a substantially saw blade shape.

In addition, in the oil well pipe of the present disclosure, in the thread shape of the first internal thread portion and/or the second internal thread portion, a flank angle of a flank surface that receives a pressure when the oil well pipe is pulled to be separated along an axial direction from a state where the oil well pipe is screwed to the different member is equal to or larger than a right angle with respect to an axial core of the oil well pipe.

In addition, in the oil well pipe of the present disclosure, a region along an axial direction is longer in the second internal thread portion than in the first internal thread portion.

In addition, in the oil well pipe of the present disclosure, the internal thread portion has a third internal thread portion that is disposed on the opening end side with respect to the second internal thread portion and has a diameter of a predetermined dimension.

In addition, in the oil well pipe of the present disclosure, the third internal thread portion has an effective diameter that is equal to or larger than an effective diameter of the second internal thread portion and has a straight thread shape.

In addition, in the oil well pipe of the present disclosure, a region along an axial direction is longer in the second internal thread portion than in the first internal thread portion and/or the third internal thread portion.

In addition, in the oil well pipe of the present disclosure, a region along an axial direction is shorter in the second internal thread portion than in the first internal thread portion and/or the third internal thread portion.

In addition, in the oil well pipe of the present disclosure, the internal thread portion has a fourth circular region including the first internal thread portion, a fifth circular region including the second internal thread portion, and a sixth circular region including the third internal thread portion that are virtually divided in a radial direction, an area of the fourth circular region is equal to or smaller than a third of a total cross-sectional area of a transverse section of the oil well pipe, an area of the fifth circular region is equal to or larger than a third of the total cross-sectional area of the transverse section of the oil well pipe, and an area of the sixth circular region is equal to or smaller than a third of the total cross-sectional area of the transverse section of the oil well pipe.

In addition, in the oil well pipe of the present disclosure, the first internal thread portion and the third internal thread portion have a thread having a symmetrical shape.

In addition, in the oil well pipe of the present disclosure, on the opening side and/or an inner side of the internal thread portion, an inner circumferential surface that can be brought into close contact with an outer circumferential surface of the different member substantially throughout an entire circumference when the internal thread portion has been fitted into the external thread portion of the different member is provided.

In addition, in the oil well pipe of the present disclosure, on the opening side and/or an inner side of the internal thread portion, an inner circumferential surface that can be brought into close contact with an outer circumferential surface of the different member substantially throughout an entire circumference when the internal thread portion has been fitted into the external thread portion of the different member is provided, and a length in an axial direction of the inner circumferential surface on the opening side of the internal thread portion is set to shorter than a length in the axial direction of the third internal thread portion and/or a length in the axial direction of the inner circumferential surface on the inner side of the internal thread portion is set to shorter than a length in the axial direction of the first internal thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, when the first oil well pipe and the second oil well pipe are in a state of being coupled to each other, the second external thread portion and the second internal thread portion interfere with each other in a radial direction, and one of the second external thread portion and the second internal thread portion elastically deforms and/or plastically deforms in the radial direction, and the external thread portion has an interference mitigation portion in a boundary region between the first external thread portion and the second external thread portion.

In addition, in the oil well pipe coupling structure of the present disclosure, when the first oil well pipe and the second oil well pipe are in a state of being coupled to each other, the second external thread portion and the second internal thread portion interfere with each other in a radial direction, and one of the second external thread portion and the second internal thread portion elastically deforms and/or plastically deforms in the radial direction, and the internal thread portion has an interference mitigation portion in a boundary region between the second internal thread portion and the third internal thread portion.

In addition, in the oil well pipe of the present disclosure, the external thread portion has an interference mitigation portion in a boundary region between the first external thread portion and the second external thread portion.

In addition, in the oil well pipe of the present disclosure, the internal thread portion has an interference mitigation portion in a boundary region between the second internal thread portion and the third internal thread portion.

According to the present disclosure, the oil well pipes can be connected together with a simple structure without increasing the size of the well pipes and with the strength significantly exceeding the limit of the prior art within the range of the inner/outer diameter of the well pipes.

In addition, it is possible to form a sealing structure that maintains a high sealing performance at high temperatures or under environments in which the oil well pipes receive large loads or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing two oil well pipes that have a coupling structure of a first embodiment and can be coupled to each other.

FIG. 2 is a cross-sectional view showing the oil well pipes of the first embodiment.

FIG. 3A and FIG. 3B are cross-sectional views showing the coupled oil well pipes.

FIG. 4 is a view showing thread of an external thread.

FIG. 5 is a view showing a different shape example of the thread.

FIG. 6 is a cross-sectional view showing the oil well pipes.

FIG. 7 is a cross-sectional view showing a different shape example of the oil well pipes.

FIG. 8A to FIG. 8C are views showing examples of a taper shape of a second external thread portion.

FIG. 9 is a cross-sectional view showing a different shape example of the oil well pipes.

FIG. 10A to FIG. 10D are views showing examples of different thread shape.

FIG. 11A to FIG. 11D are views showing examples of different thread shape.

FIG. 12 is a view showing a sealing structure between an external thread portion and an internal thread portion.

FIG. 13A and FIG. 13B show different shape examples of the oil well pipe having the external thread portion, FIG. 13A is a cross-sectional view, and FIG. 13B is a view showing circular regions A to C.

FIG. 14 is a view showing an outer circumferential surface of an oil well pipe having an external thread portion.

FIG. 15A and FIG. 15B show different shape examples of the oil well pipe having the internal thread portion, FIG. 15A is a cross-sectional view, and FIG. 15B is a view showing circular regions D to F.

FIG. 16A and FIG. 16B are views showing position examples of a seal portion.

FIG. 17A to FIG. 17C are views showing examples of a tip portion.

FIG. 18A and FIG. 18B are views showing a length in an axial direction of each portion of each oil well pipe.

FIG. 19A and FIG. 19B are views cross-sectional views showing different shape examples of the oil well pipe.

FIG. 20 is a graph showing results of tensile tests and compressive tests of the oil well pipes.

FIG. 21 is a view showing an interference mitigation portion provided in the external thread portion.

FIG. 22 is a cross-sectional view showing a different shape example of the oil well pipes.

FIG. 23 is a view showing an interference mitigation portion provided in the internal thread portion.

FIG. 24A and FIG. 24B are views showing positions of the interference mitigation portions in the oil well pipes.

DETAILED DESCRIPTION

Hereinafter, an embodiment of an oil well pipe coupling structure of the present disclosure will be described with reference to drawings. FIG. 1 is a perspective view showing two oil well pipes 1 and 1A that have an oil well pipe coupling structure of the first embodiment and can be coupled together. FIG. 2 is a cross-sectional view showing the oil well pipes 1 and 1A of the first embodiment. FIGS. 1 and 2 show a main part for coupling the oil well pipes 1 and 1A, but do not show the entirety. That is, FIGS. 1 and 2 show one end portion in which an external thread portion of the oil well pipe 1 is formed and one end portion in which an internal thread portion of the oil well pipe 1A is formed. In addition, the outer diameter of the external thread portion and the inner diameter of the internal thread portion, which will be described below, are set so that the dimensions of the outer diameters of the oil well pipes 1 and 1A become substantially uniform when the oil well pipes 1 and 1A are connected together.

The oil well pipes 1 and 1A are, for example, so-called steel pipes or the like having a hollow structure in which the dimension of the outer diameter is substantially uniform, thorough which a through hole 2 is provided in the axial direction so that fluids can be transferred. Therefore, the oil well pipes 1 and 1A are used in wells for minable resource fluids such as crude oil wells and natural gas wells in mining sites.

The oil well pipe 1 has one end portion in which the outer diameter is reduced and an external thread portion 10 on the outer circumferential surface of the one end portion. The external thread portion 10 has a first external thread portion 12 with a straight thread in which the outer diameter of the external thread is substantially constant and a second external thread portion 14 that is disposed on the one end portion side with respect to the first external thread portion 12 and has a so-called tapered thread in which the outer diameter of the external thread is gradually reduced. In addition, a region along the axial direction is set to be longer in the second external thread portion 14 than in the first external thread portion 12.

In the second external thread portion 14, the thread are formed so that the outer diameter of the external thread gradually decreases toward the one end portion. Specifically, the thread in the second external thread portion 14 are formed at the same pitches as thread in the first external thread portion 12 so that the outer diameter and the thread heights gradually decrease toward the one end side. The thread in the first external thread portion 12 and the thread in the second external thread portion 14 are continuously formed at the same pitches, but the configuration is not limited thereto, and the pitches in any one of the external thread portions may be set to be larger or smaller than the pitches in the other or the thread may be configured so that the pitches gradually change.

In the oil well pipe 1A, the inner circumferential surface of one end portion radially expands, and an internal thread portion 20 is provided on the inner circumferential surface. The internal thread portion 20 has a first internal thread portion 22 and a second internal thread portion 24, and the first internal thread portion 22 is position on the one end side with respect to the second internal thread portion 24. In addition, a region along the axial direction is set to be longer in the second internal thread portion 24 than in the first internal thread portion 22.

The first internal thread portion 22 has a straight thread in which the diameter of the internal thread is substantially constant. The second internal thread portion 24 has a tapered thread in which the diameter of the thread gradually changes. Specifically, in the second internal thread portion 24, the thread are formed so that the inner diameter of the internal thread gradually increases toward the one end side (that is, the first internal thread portion 22 side). In addition, the pitches of the thread in the second internal thread portion 24 are set to be the same as the pitches of thread in the first internal thread portion 22. As a result, the thread in the internal thread portion 20 are formed so that a straight thread is formed in the first internal thread portion 22 and a tapered thread, in which the inner diameter is gradually reduced from the first internal thread portion 22 (one end) side, is formed in the second internal thread portion 24.

The thread in the first internal thread portion 22 and the thread in the second internal thread portion 24 are continuously formed at the same pitches, but the configuration is not limited thereto, and the pitches in any one of the internal thread portions may be set to be larger or smaller than the pitches in the other or the thread may be configured so that the pitches gradually change.

In addition, the thread shapes in the external thread portion 10 and in the internal thread portion 20 is not particularly limited and may be, for example, a saw blade shape. Here, FIG. 4 is a view showing the thread shape in the external thread portion 10, and, as shown in FIG. 4, regarding the flank angle θ of a flank surface 4 of the thread, on which a pressure is exerted when the oil well pipes 1 and 1A are pulled to be separated from each other in the axial direction from a state where the oil well pipes 1 and 1A are screwed and connected together, it is set to substantially a right angle with respect to the axial core of the oil well pipe 1. Here, the flank angle θ refers to an external angle in a cross section of the triangular thread.

The thread shape in the internal thread portion 20 is set to the same shape as in the external thread portion 10. That is, in the internal thread portion 20, the flank angle of a flank surface that comes into contact with the flank surface 4 of the external thread portion 10 is set to a flank angle θ that is the same flank angle as in the external thread portion 10. It is needless to say that the flank angle may be set to an angle other than the right angle as long as the external thread portion 10 and the internal thread portion 20 are configured so that the flank surfaces come into contact with each other throughout the entire surfaces, and, for example, the flank angle θ in the external thread portion 10 may be set to an angle larger than the right angle or, conversely, the flank angle θ may be set to a so-called lean-back shape in which the angle is smaller than the right angle as shown in FIG. 5.

The oil well pipes 1 and 1A are connected to each other by screwing the external thread portion 10 and the internal thread portion 20 together. Specifically, the one end portion of the oil well pipe 1 is inserted into the inner circumferential surface of the oil well pipe 1A on the one end portion side. At this time, first, the second external portion 14 present on the tip side of the oil well pipe 1 is passed through the inner circumferential surface of the oil well pipe 1A, which is surrounded by the first internal thread portion 22 on the opening side, and the oil well pipe 1 is disposed at a position where the first external thread portion 12 shown in FIG. 3A can be screwed into the first internal thread portion 22. Next, the oil well pipe 1 is rotated in a fastening direction of the thread with respect to the oil well pipe 1A so that the first external thread portion 12 is screwed into the first internal thread portion 22.

Therefore, the oil well pipe 1 is gradually moved up to a position where the second external thread portion 14 can be screwed into the second internal thread portion 24 and, furthermore, rotated to be in a state where the external thread of the second external thread portion 14 and the internal thread of the second internal thread portion 24 are screwed together as shown in FIG. 3B.

Therefore, the oil well pipes 1 and 1A are connected to each other in a state where the external thread portion 10 and the internal thread portion 20 are screwed together. That is, the oil well pipes 1 and 1A are connected to each other in a state where the first external thread portion 12 is screwed into the first internal thread portion 22 and the second external thread portion 14 is screwed into the second internal thread portion 24. In addition, when the external thread portion 10 and the internal thread portion 20 are in a state of being fitted with each other, a virtual line connecting the peaks of the thread in the external thread portion 10 that are arranged in the axial direction and a virtual line connecting the peaks of the thread in the internal thread portion 20 that are arranged in the axial direction become parallel to each other. That is, the virtual line in the first external thread portion 12 and the virtual line in the first internal thread portion 22 are parallel to each other, and the virtual line in the second external thread portion 14 and the virtual line in the second internal thread portion 24 are parallel to each other.

Here, the strength in the axial direction of the oil well pipes 1 and 1A connected to each other differs at a place where the first external thread portion 12 and the first internal thread portion 22 are screwed together and at a place where the second external thread portion 14 and the second internal thread portion 24 are screwed together.

First, in the first external thread portion 12 of the oil well pipe 1, a load concentrates at a place where the thickness of the pipe is thinnest near the beginning point of the thread, that is, around a position P1 in FIG. 6. Therefore, the strength in the first external thread portion 12 depends on the strength at the position P1, and the percentage of the strength at the position P1 with respect to the total cross-sectional strength is substantially equivalent to the percentage of the cross-sectional area at the position P1 with respect to the total cross-sectional area. For example, when the percentage of the cross-sectional area at the position P1 with respect to the total cross-sectional area at a position PO is approximately 90%, the strength becomes approximately 90% of the total cross-sectional strength.

Since the second external thread portion 14 of the oil well pipe 1 has a tapered thread in which the outer diameter changes toward the tip, the oil well pipe 1 is tightly screwed in a section in which the thread is effectively engaged with the thread in the second internal thread portion 24 of the oil well pipe 1A. That is, the strength corresponds to a shear strength relating to the shear cross-sectional area of the thread, which depends on the thread engagement effective length. In the second external thread portion 14, a load is dispersed to the thread in the thread engagement effective length, and consequently, a strength that is approximately 70% of the total cross-sectional strength can be obtained in a case where the second external thread portion 14 is set so that the thread engagement effective length is maximized.

In the first internal thread portion 22 and the second internal thread portion 24 of the oil well pipe 1A as well, the strengths are determined in substantially the same manner as in the first external thread portion 12 and the second external thread portion 14 of the oil well pipe 1. That is, in the first internal thread portion 22, a load concentrates at a place where the thickness is thinnest near the beginning point of the thread (a position P3 in FIG. 6). Therefore, the strength in the first internal thread portion 22 is determined by the percentage of the cross-sectional area at the position P3 where the thickness is thinnest with respect to the total cross-sectional area (the cross-sectional area at a position P4 in FIG. 6) and the total cross-sectional strength. In addition, since the second internal thread portion 24 has a tapered thread, a strength of a maximum of approximately 70% of the total cross-sectional strength can be obtained.

In the external thread portion 10 and the internal thread portion 20, since the strength differs in the region where the straight thread is disposed and, in the region, where the tapered thread is disposed as described above, the tensile strength of all of the thread portions is determined by the sum of the tensile strengths in the respective regions. Specifically, the tensile strength of the external thread portion 10 is determined by the sum of the tensile strength in the first external thread portion 12 and the tensile strength in the second external thread portion 14, and, when the tensile strength in the first external thread portion 12 region is approximately 10% of the tensile strength of the total cross section and the tensile strength in the second external thread portion 14 region is approximately 70% of the tensile strength of the total cross section as in the above-described example, the tensile strength of the entire external thread portion 10 becomes approximately 80% of the tensile strength of the total cross section. In the internal thread portion 20 as well, the sum of the tensile strength in the first internal thread portion 22 region and the tensile strength in the second internal thread portion 24 region becomes the tensile strength of the entire internal thread portion 20.

As described above, the external thread portion and the internal thread portion are provided in the oil well pipes 1 and 1A, respectively, the straight thread of the first external thread portion and the first internal thread portion are screwed together, and the tapered thread of the second external thread portion and the second internal thread portion are screwed together, whereby it is possible to improve the strength in the axial direction compared with a case where an external thread and an internal thread are screwed together simply with a straight thread or a case where an external thread and an internal thread are screwed together simply with a tapered thread.

In the above-described embodiment, the external thread portion 10 is formed so that the first external thread portion 12 and the second external thread portion 14 are disposed in this order from a middle portion of the oil well pipe 1 toward the one end portion, but the external thread portion 10 may be formed so that the first external thread portion 12, the second external thread portion 14, and a third external portion 16 with a straight thread are disposed in this order from the middle portion toward the one end portion as shown in FIG. 7.

In addition, in the internal thread portion 20, the first internal thread portion 22 and the second internal thread portion 24 are disposed in this order from the one end of the oil well pipe 1A, but the internal thread portion 20 may be formed so that the first internal thread portion 22, the second internal thread portion 24, and a third internal portion 26 with a straight thread are disposed in this order from the one end as shown in FIG. 7.

In addition, usually, at the time of connecting oil well pipes, the pipes are connected to each other via a separate joint in order to improve the strength. Since the outer diameter of the joint is larger than the diameters of the oil well pipes, a well, a hole, or the like with a larger diameter than the joint portion has been provided in accordance with the joint diameter in order to install such oil well pipes. However, according to the oil well pipe of the present disclosure, since an increase in the dimension of the outer diameter of the pipe can be suppressed while the pipe has an extremely high strength in the axial direction, it is possible to reduce time and effort taken to excavate wells, holes, or the like and to reduce the cost for excavation.

In addition, in the oil well pipes 1 and 1A connected together, since the straight thread in the first external thread portion and the first internal thread portion are screwed together, and the tapered thread in the second external thread portion and the second internal thread portion are screwed together, compared with a case where steel pipes with a flange are connected together, no flange that protrudes in the radial direction is installed, and the strength in the axial direction can be improved. In addition, since no flange is provided, it is possible to suppress an increase in the sizes of the oil well pipes 1 and 1A and to decrease a space necessary for the disposition of the oil well pipes 1 and 1A, which enables spaces to be saved. In addition, even in the case of storing the oil well pipes 1 and 1A in an arrayed state, it is possible to save a space for the storage place.

In addition, since there is no case where fluids leak due to the formation of a gap between flange joint surfaces, it is possible to more stably transfer fluids.

In the above-described embodiment, the taper shapes of the tapered thread in the second external thread portion 14 and the second internal thread portion 24 can be appropriately set. For example, the taper shape can be set to a taper shape in which, in a cross-sectional shape of the external thread portion 10 shown in FIG. 8A, a virtual line connecting the tips of individual thread in the second external thread portion 14 is linear and inclined at a predetermined gradient or can be set to a taper shape in which a virtual line connecting the tips of individual thread in the second external thread portion 14 shown in FIG. 8B is inclined in a curved shape. In addition, it is also possible to set the taper shape to a curved shape in which a virtual line connecting the tips and/or root portions of individual thread in the second external thread portion 14 shown in FIG. 8C along the axial direction has a tangent curve shape. It is needless to say that, in the second internal thread portion 24 of the internal thread portion 20 as well, the tapered thread can be set, similar to the above-described tapered thread, to a taper shape inclined at a predetermined gradient or to a taper shape inclined in a curved shape.

In addition, in the above-described embodiment, the thread is provided continuously to form the first external thread portion 12 and the second external thread portion 14, but the configuration is not limited thereto, and the thread in the first external thread portion 12 and the thread in the second external thread portion 14 may be separately formed. In addition, a non-threaded portion 18 may be provided between the first external thread portion 12 and the second external thread portion 14 as shown in FIG. 9. The outer diameter of the non-threaded portion 18 is set to equal to or smaller than the root diameter of the thread in at least the first external thread portion 12. That is, the non-threaded portion 18 is set to have an outer diameter at which the non-threaded portion 18 does not interfere with the thread in the internal thread portion 20 and does not hinder screwing at the time of screwing the external thread portion 10 and the internal thread portion 20. In addition, the non-threaded portion 18 desirably has an outer diameter that is constant along the axial direction.

In addition, in a case where the non-threaded portion 18 is disposed in the external thread portion 10, similarly, a non-threaded portion may be disposed in the internal thread portion 20. In addition, to the non-threaded portion 18, a metal sealing structure may be provided or a sealing material such as an O ring, a D ring, or a gasket may be attached. When a sealing material is interposed between the external thread portion 10 and the internal thread portion 20, the sealing property further improves, and it is possible to prevent the leakage of fluids that flow in the pipes or the like.

In addition, the thread shapes in the external thread portion 10 and in the internal thread portion 20 are not limited to a saw blade shape, can be appropriately set, and can be, for example, a triangular thread shape shown in FIG. 10A, a round thread shape shown in FIG. 10B, a square thread shape shown in FIG. 10C, a trapezoidal thread shape shown in FIG. 10D, or the like. In addition, the thread shape can be a saw blade shape in which the tip surface of the thread is formed wide as shown in FIG. 11A, a lean-back shape in which the tip surface of the thread is formed wide as shown in FIG. 11B, or the like. In addition, the thread shape may be a saw blade shape in which the tip surface of the thread or the root portion of the thread is curved as shown in FIG. 11C or a lean-back shape in which the tip surface of the thread or the root portion of the thread is curved as shown in FIG. 11D.

In the external thread portion 10, the thread shapes may be different in the first external thread portion and in the second external thread portion, and it is also possible to set, for example, the thread in the first external thread portion, which is a straight thread, to a triangular shape and the thread in the second external thread portion, which is a tapered thread, to a saw blade shape or the like. In addition, this is also true for the external thread portion having the first external thread portion to the third external thread portion as shown in FIG. 7, and it is also possible to set the thread in the first external thread portion to a triangular shape, the thread in the second external thread portion to a saw blade shape, and the thread in the third external thread portion to a round thread shape or the like.

In addition, the outer diameter of the external thread portion 10 and the inner diameter of the internal thread portion 20 can be appropriately set. Therefore, the outer diameter of the external thread portion 10 and/or the inner diameter of the internal thread portion 20 may be set in order to improve the sealing property between the external thread portion 10 and the internal thread portion 20. That is, the sealing property may be improved by causing the second external thread portion 14 to press the second internal thread portion 24 outward from the inside to elastically deform and/or plastically deform at least one of the second external thread portion 14 and the second internal thread portion 24.

For example, the maximum outer diameter (or the maximum effective diameter) of the second external thread portion 14 is set to be larger than the maximum inner diameter (or the maximum effective diameter) of the second internal thread portion 24 and/or the minimum outer diameter (or the minimum effective diameter) of the second external thread portion 14 is set to be larger than the minimum inner diameter (or the minimum effective diameter) of the second internal thread portion 24 so that the external thread portion 20 is capable of slightly expanding outward in the radial direction in a state where the external thread portion 10 and the internal thread portion 20 are fully screwed together. In such a case, the second external thread portion 14 and the second internal thread portion 24 are capable of interfering each other in the radial direction at least in a part, and, at the time of screwing the external thread portion 10 and the internal thread portion 20, the internal thread portion 20 radially expands while elastically deforming or plastically deforming, consequently, the external thread portion 10 and the internal thread portion 20 come into close contact with each other, and thereby the sealing property improves.

It is needless to say that the dimension of the inner diameter of the second internal thread portion 24 with respect to the outer diameter of the second external thread portion 14 may be set so that the second external thread portion 14 elastically deforms and/or plastically deforms inward in the radial direction. In addition, in the above description, the outer diameter of the second external thread portion 14 and the inner diameter of the second internal thread portion 24 are set; however, additionally, the thicknesses of the pipes in the second external thread portion 14 and/or in the second internal thread portion 24 may be set.

In addition, in a case where the second external thread portion 14 and the second internal thread portion 24 are configured to interfere with each other, when the external thread portion 10 and the internal thread portion 20 are screwed together up to a regular position (screwed together deepest), the tip of the thread in the first external thread portion 12 near the second external thread portion 14 is positioned on the outer side with respect to the thread root portion in the second internal thread portion 24 near the first internal thread portion 22-side end portion in the radial direction and interferes with the thread root portion. In such an interference state, it becomes impossible to further screw the external thread portion 10 and the internal thread portion 20. Therefore, the end portion region in the diameter of the first external thread portion 12 that interferes with the second internal thread portion 24 is configured to radially contract, thereby decreasing or removing the degree of interference.

The length of the radial contraction is preferably set to a length that is equal to or longer than the interference length between the end portion of the first external thread portion 12 and the second internal thread portion 24. In addition, the end portion region of the first external thread portion 12 is radially contracted by providing a variety of shapes such as a stair shape, a taper shape, a curved shape, and a non-threaded shape to the end portion region.

Specifically, since the first external thread portion 12 is capable of interfering with the second internal thread portion 24 in the axial direction, in order to avoid such interference, an interference mitigation portion 13 as shown in FIG. 21 is provided in an outwardly projecting (that is, projecting outward in the radial direction) boundary region between the first external thread portion 12 and the second external thread portion 14.

For example, a virtual line 13A connecting the tips of individual thread in the interference mitigation portion 13 (refer to FIG. 21) forms a curved shape that is closer to the axial core than a linear virtual line 12 a connecting the tips of individual thread in the first external thread portion 12 and a tapered virtual line 14 a connecting the tips of individual thread in the second external thread portion 14, and the heights or positions of the thread are determined so as to be along the curved line. In addition, the radius r that is maximized in the interference mitigation portion 13 (refer to FIG. 21) is equal to or smaller than the minimum inner diameter of the second internal thread portion 24.

Therefore, interference between the first external thread portion 12 and the second internal thread portion 24 can be mitigated or avoided by providing the interference mitigation portion 13. In addition, the interference mitigation portion 13 may have a surface shape so as to mitigate or avoid the interference with the second internal thread portion 24 and also may have a same thread that can be screwed into the first internal thread portion 22 and/or the second internal thread portion 24.

The external thread portion 10 has been described to have a shape in which the first external thread portion 12 with a straight thread and the second external thread portion 14 with a tapered thread are provided from the middle portion of the oil well pipe 1 toward the one end portion, but it is needless to say that the external thread portion 10 may have a shape in which the second external thread portion 14 and the first external thread portion 12 are provided from the middle portion toward the one end portion as shown in FIG. 22. In such a case, the internal thread portion 20 that can be screwed to the external thread portion 10 has a shape in which the second internal thread portion 24 and the first internal thread portion 22 are provided from the one end shown in FIG. 22 toward the inside. In addition, the external thread portion 10 may have a seal portion 19 that forms an annular shape on the middle portion side of the second external thread portion 14 and is capable of coming into close contact with the inner circumferential surface of the oil well pipe 1A throughout the entire circumference.

In addition, in the oil well pipes 1 and 1A in FIG. 22, in a case where the second external thread portion 14 and the second internal thread portion 24 are configured to interfere with each other, when the external thread portion 10 and the internal thread portion 20 are screwed together up to the regular position (screwed together deepest), the first external thread portion 12-side end portion of the second external thread portion 14 interferes with a region of the first internal thread portion 22 near the second internal thread portion 24-side end portion. Therefore, the end portion region in the first internal thread portion 22 that interferes with the second external thread portion 14 is configured to radially expand, thereby decreasing or removing the degree of interference.

The length of the radial expansion is preferably set to a length that is equal to or longer than the interference length between the end portion of the first internal thread portion 22 and the second external thread portion 14. In addition, the end portion region of the first internal thread portion 22 is radially expanded by providing a variety of shapes such as a stair shape, a taper shape, a curved shape, and a non-threaded shape to the end portion region.

Specifically, an interference mitigation portion 23 (refer to FIG. 23) is provided in an outwardly projecting (that is, projecting inward in the radial direction) boundary region between the first internal thread portion 22 and the second internal thread portion 24. For example, a virtual line 23 a connecting the tips of individual thread in the interference mitigation portion 23 (refer to FIG. 23) forms a curved shape that is positioned on the outer side of the axial core with respect to a virtual line 22 a connecting the tips of individual thread in the first internal thread portion 22 and a tapered virtual line 24 a connecting the tips of individual thread in the second internal thread portion 24, and the heights or positions of the thread are determined so as to be along the curved line. In addition, the radius R that is maximized in the interference mitigation portion 23 (refer to FIG. 23) is equal to or larger than the minimum outer diameter of the second external thread portion 14. Interference between the first internal thread portion 22 and the second external thread portion 14 can be mitigated or avoided by providing the interference mitigation portion 23. In addition, the interference mitigation portion 23 may have a surface shape so as to mitigate or avoid the interference with the second external thread portion 14 and also may have a same thread that can be screwed into the first external thread portion 12 and/or the second external thread portion 14.

In addition, in order to dispose a sealing structure between the external thread portion 10 and the internal thread portion 20, as shown in FIG. 12, an outer circumferential surface 30 of the tip portion of the external thread portion 10 of the oil well pipe 1 may be configured to be brought into close contact with an inner circumferential surface 40 on the base end side of the internal thread portion 20 of the oil well pipe 1A. That is, in the insertion direction of the oil well pipe 1 into the oil well pipe 1A, the outer circumferential surface 30 that is positioned on the front side in the insertion direction of the external thread portion 10 may be brought into close contact with the inner circumferential surface 40 that is positioned on the rear side in the insertion direction into the oil well pipe 1A.

In such a case, the outer diameter in the outer circumferential surface 30 is set to be slightly larger than the inner diameter in the inner circumferential surface 40. Therefore, when the external thread portion 10 and the internal thread portion 20 have been fitted with together, an elastic force is exerted so that the outer circumferential surface 30 presses the inner circumferential surface 40 outward or the inner circumferential surface 40 presses the outer circumferential surface 30 inward. As a result, the outer circumferential surface 30 and the inner circumferential surface 40 come into close contact with each other throughout the entire circumference and exhibit sealing performance.

In addition, an outer circumferential surface 32 on the base end portion of the external thread portion 10 of the oil well pipe 1 may be configured to be brought into close contact with an inner circumferential surface 42 on the tip side of the internal thread portion 20 of the oil well pipe 1A. In such a case, the outer diameter in the outer circumferential surface 32 is set to be slightly larger than the inner diameter in the inner circumferential surface 42.

Even in this case, when the external thread portion 10 and the internal thread portion 20 have been fitted with together, an elastic force is exerted so that the outer circumferential surface 32 presses the inner circumferential surface 42 outward or the inner circumferential surface 42 presses the outer circumferential surface 32 inward, and the outer circumferential surface 32 and the inner circumferential surface 42 come into close contact with each other throughout the entire circumference and exhibit sealing performance. The outer circumferential surfaces 30 and 32 and the inner circumferential surfaces 40 and 42 may be inclined surfaces that are inclined with respect to the axial core of the external thread portion 10 and the internal thread portion 20.

Next, oil well pipes according to different configurations will be described. FIG. 13A and FIG. 13B show different configuration examples of an oil well pipe having an external thread portion, FIG. 13A is a cross-sectional view, and FIG. 13B is a view showing circular regions A to C. In an oil well pipe 300, a first seal portion 310, an external thread portion 320, and a second seal portion 330 are sequentially disposed from the tip side that is connected with an oil well pipe 400. In addition, the oil well pipe 300 has a tip portion 300 a that regulates the insertion depth into the oil well pipe 400.

As shown in FIG. 14, the first seal portion 310 has at least one annular projecting portion 312 that extends in the circumferential direction on the entire outer circumferential surface and protrudes outward in the radial direction and preferably has a plurality of the projecting portions 312. In addition, in the first seal portion 310, the annular projecting portions 312 are arranged in parallel in the axial direction, thereby defining an annular recessed portion 314 forming a shape that is relatively recessed with respect to the annular projecting portions 312 between the annular projecting portions 312. The projecting length of the annular projecting portion 312 is set to a length at which at least the screwing between a first external thread portion 322 and a first internal thread portion 422, which will be described below (refer to FIG. 15A), is not hindered and the annular projecting portion 312 is capable of coming into close contact with (or slightly interfering with) the inner circumferential surface of a first non-threaded portion 410 (refer to FIG. 15A).

The external thread portion 320 is formed by disposing a first external thread portion 322 with a straight thread, a second external thread portion 324 with a tapered thread, and a third external thread portion 326 with a straight thread sequentially from the base end side, the effective diameter of the second external thread portion 324 is equal to or smaller than the effective diameter of the first external thread portion 322, and the effective diameter of the third external thread portion 326 is equal to or smaller than the effective diameter of the second external thread portion 324. The effective diameter of the second external thread portion 324 is larger on the base end side than on the tip side. Here, the thread in the first external thread portion 322 through the third external thread portion 326 are configured to form a continuous spiral shape so that the maximum effective diameter of the second external thread portion 324 substantially corresponds to the effective diameter of the first external thread portion 322 and the minimum effective diameter substantially corresponds to the effective diameter of the third external thread portion 326.

The oil well pipe 300 has concentric circular region A (first circular region), circular region B (second circular region), and circular region C (third circular region) that include any of the first external thread portion 322 through the third external thread portion 326 and are virtually divided into three parts in the radial direction when seen in the axial direction. As shown in FIG. 13B, a region in the radial direction from the outer circumferential surface of the oil well pipe 300 to the effective diameter portion of the first external thread portion 322 is defined as the circular region A. In addition, a region in the radial direction from the maximum effective diameter portion of the second external thread portion 324 (that is, the effective diameter portion of the first external thread portion 322) to the minimum effective diameter portion of the second external thread portion 324 is defined as the circular region B. In addition, a region in the radial direction from the effective diameter portion of the third external thread portion 326 (that is, the minimum effective diameter portion of the second external thread portion 324) to the inner circumferential surface of the oil well pipe 300 is defined as the circular region C.

The area of the circular region A is set to a third or less of the total cross-sectional area of a transverse section of the oil well pipe 300. In addition, the area of the circular region B is set to a third or more of the total cross-sectional area of the transverse section of the oil well pipe 300. In addition, the area of the circular region C is set to a third or less of the total cross-sectional area of the transverse section of the oil well pipe 300.

In the first external thread portion 322, the shape or the like of the threaded region is set so that the shear area of the entire region of the thread that are screwed and fitted with the first internal thread portion 422 becomes equal to or larger than a dimension obtained by multiplying the circular region A area by root 3. Therefore, the thread engagement effective length, the number of the thread, the pitches, and the like are set so that the total shear area of the thread in the first external thread portion 322 that are formed by continuously shearing the thread in a spiral shape at the effective diameter portion of the thread becomes equal to or larger than double root 3 of the circular region A area.

In addition, in the second external thread portion 324, the shape or the like of the threaded region is set so that the shear area of the entire region of the thread that are screwed and fitted with a second internal thread portion 424 becomes equal to or larger than a dimension obtained by multiplying the circular region B area by root 3. Therefore, the thread engagement effective length, the number of the thread, the pitches, and the like are set so that the total shear area of the thread in the second external thread portion 324 that are formed by continuously shearing the thread in a spiral shape at the effective diameter portion of the thread becomes equal to or larger than double root 3 of the circular region B area.

In addition, in the third external thread portion 326, the shape or the like of the threaded region is set so that the shear area of the entire region of the thread that are screwed and fitted with a third internal thread portion 426 becomes equal to or larger than a dimension obtained by multiplying the circular region C area by root 3. Therefore, the thread engagement effective length, the number of the thread, the pitches, and the like are set so that the total shear area of the thread in the third external thread portion 326 that are formed by continuously shearing the thread in a spiral shape at the effective diameter portion of the thread becomes equal to or larger than double root 3 of the circular region C area.

The second seal portion 330 has, similar to the first seal portion 310, one or more circular projecting portions 332 and preferably has a plurality of the circular projecting portions 332, and an annular recessed portion 334 is defined between the annular projecting portions 332. In addition, the projecting length of the annular projecting portion 332 is set to a length at which the annular projecting portion 332 is capable of coming into close contact with (or slightly interfering with) the inner circumferential surface of a second non-threaded portion 430 (refer to FIG. 15A).

The tip portion 300 a is more preferably a tip portion that comes into contact with the oil well pipe 400 and has a shape capable of regulating the position in the depth direction, preferably, a guidable shape that facilitates insertion and can be formed by, for example, chamfering the tip of the first seal portion 310 as shown in FIG. 14. In addition, as regulation means that comes into contact with the oil well pipe 400 and is capable of regulating the position in the depth direction, a base end portion 300 b may be formed on the base end side of the second seal portion 330 in place of the tip portion 300 a or together with the tip portion 300 a.

In addition, FIG. 15A and FIG. 15B show different configuration examples of an oil well pipe having an internal thread portion, FIG. 15A is a cross-sectional view, and FIG. 15B is a view showing circular regions D to F. The oil well pipe 400 has a first non-threaded portion 410, an internal thread portion 420, and a second non-threaded portion 430. When the oil well pipe 400 is connected with the oil well pipe 300, the first non-threaded portion 410 corresponds to the first seal portion 310, and the second non-threaded portion 430 corresponds to the second seal portion 330.

In addition, the oil well pipe 400 has a receiving portion 400 a that regulates the insertion of the oil well pipe 300, and the receiving portion 400 a is disposed at a place corresponding to the tip portion 300 a (or the base end portion 300 b). The first non-threaded portion 410 has an inner circumferential surface having a circumferential shape in which the inner diameter is constant in the axial direction, and the inner diameter is set to smaller than the diameter in the third internal thread portion 426 described below. In addition, the second non-threaded portion 430 has an inner circumferential surface having a circumferential shape in which the inner diameter is constant in the axial direction, and the inner diameter is set to equal to or larger than the root diameter in the first internal thread portion 422 described below.

The internal thread portion 420 is formed by disposing the first internal thread portion 422 with a straight thread, the second internal thread portion 424 with a tapered thread, and the third internal thread portion 426 with a straight thread sequentially from one end side into which the oil well pipe 300 is inserted. At least the minimum effective diameter of the second internal thread portion 424 is equal to or larger than the effective diameter of the first internal thread portion 422, and the effective diameter of the third internal thread portion 426 is equal to or larger than the maximum effective diameter of the second internal thread portion 424. The first internal thread portion 422 is screwed to the first external thread portion 322, the second internal thread portion 424 is screwed to the second external thread portion 324, and the third internal thread portion 426 is screwed to the third external thread portion 326.

The oil well pipe 400 has concentric circular region D (fourth circular region), circular region E (fifth circular region), and circular region F (sixth circular region) that include any of the first internal thread portion 422 through the third internal thread portion 426 and are virtually divided into three parts in the radial direction when seen in the axial direction. As shown in FIG. 15B, a region in the radial direction from the outer circumferential surface of the oil well pipe 400 to the effective diameter portion of the first internal thread portion 422 is defined as the circular region D. In addition, a region in the radial direction from the maximum effective diameter portion of the second internal thread portion 424 (that is, the effective diameter portion of the first internal thread portion 422) to the minimum effective diameter portion of the second internal thread portion 424 is defined as the circular region E. In addition, a region in the radial direction from the effective diameter portion of the third internal thread portion 426 (that is, the minimum effective diameter portion of the second internal thread portion 424) to the inner circumferential surface of a bare pipe portion of the oil well pipe 400 is defined as the circular region F.

The area of the circular region D is set to a third or less of the total cross-sectional area of a transverse section of the oil well pipe 400. In addition, the area of the circular region E is set to a third or more of the total cross-sectional area of the transverse section of the oil well pipe 400. In addition, the area of the circular region F is set to a third or less of the total cross-sectional area of the transverse section of the oil well pipe 400. Here, the circular region D is configured to have an area substantially corresponding to the circular region A, the circular region E is configured to have an area substantially corresponding to the circular region B, and the circular region F is configured to have an area substantially corresponding to the circular region C.

In the first internal thread portion 422, the shape or the like of the threaded region is set so that the shear area of the entire region of the thread that are screwed and fitted with the first external thread portion 322 becomes equal to or larger than a dimension obtained by multiplying the circular region D area by root 3. That is, the thread engagement effective length, the number of the thread, the pitches, and the like are set so that the total shear area of the thread in the first internal thread portion 422 that are formed by continuously shearing the thread in a spiral shape at the effective diameter portion of the thread becomes equal to or larger than double root 3 of the circular region D area. Here, the shape of the threaded region of the first internal thread portion 422 is set so as to correspond to the thread engagement effective length, the number of the thread, and the pitches in the first external thread portion 322, thereby satisfying the above-described conditions.

In addition, in the second internal thread portion 424, the shape or the like of the threaded region is set so that the shear area of the entire region of the thread that are screwed and fitted with the second external thread portion 324 becomes equal to or larger than a dimension obtained by multiplying the circular region E area by root 3. That is, the thread engagement effective length, the number of the thread, the pitches, and the like are set so that the total shear area of the thread in the second internal thread portion 424 that are formed by continuously shearing the thread in a spiral shape at the effective diameter portion of the thread becomes equal to or larger than double root 3 of the circular region E area. Here, the shape of the threaded region of the second internal thread portion 424 is set so as to correspond to the thread engagement effective length, the number of the thread, and the pitches in the second external thread portion 324, thereby satisfying the above-described conditions.

In addition, in the third internal thread portion 426, the shape or the like of the threaded region is set so that the shear area of the entire region of the thread that are screwed and fitted with the third external thread portion 326 becomes equal to or larger than a dimension obtained by multiplying the circular region F area by root 3. That is, the thread engagement effective length, the number of the thread, the pitches, and the like are set so that the total shear area of the thread in the third internal thread portion 426 that are formed by continuously shearing the thread in a spiral shape at the effective diameter portion of the thread becomes equal to or larger than double root 3 of the circular region F area. Here, the shape of the threaded region of the third internal thread portion 426 is set so as to correspond to the thread engagement effective length, the number of the thread, and the pitches in the third external thread portion 326, thereby satisfying the above-described conditions.

The first non-threaded portion 410 and the first seal portion 310 are substantially equal to each other in length in the axial direction, but the first non-threaded portion 410 is designed to become longer. The internal thread portion 420 and the external thread portion 320 are substantially equal to each other in length in the axial direction. The second non-threaded portion 430 and the second seal portion 330 are substantially equal to each other in length in the axial direction, but the second non-threaded portion 430 is designed to become longer.

The length in the axial direction of the first seal portion 310 is preferably set so that the first seal portion 310 does not engage with the first non-threaded portion 410 before the external thread portion 320 and the internal thread portion 420 are screwed together. Specifically, when the length in the axial direction of the first seal portion 310 shown in FIG. 18A is represented by L1 and the length in the axial direction of the third external thread portion 326 that is adjacent to the first seal portion 310 in the axial direction is represented by L2, it is desirable to set the respective lengths in the axial direction to satisfy L1<L2. Similarly, the length in the axial direction of the second seal portion 330 is preferably set so that the second seal portion 330 does not engage with the second non-threaded portion 430 before the external thread portion 320 and the internal thread portion 420 are screwed together. Specifically, when the length in the axial direction of the second seal portion 330 shown in FIG. 18A is represented by L3 and the length in the axial direction of the first external thread portion 322 that is adjacent to the second seal portion 330 in the axial direction is represented by L4, it is desirable to set the respective lengths in the axial direction to satisfy L3<L4.

The respective lengths in the axial direction of the first non-threaded portion 410 and the second non-threaded portion 430 are also preferably set in the same manner so as to correspond to the lengths in the axial direction of the first seal portion 310 and the second seal portion 330. Specifically, when the length in the axial direction of the first non-threaded portion 410 shown in FIG. 18B is represented by L1′ and the length in the axial direction of the third internal thread portion 426 that is adjacent to the first non-threaded portion 410 in the axial direction is represented by L2′, it is desirable to set the respective lengths in the axial direction to satisfy L1′<L2′. When the length in the axial direction of the second non-threaded portion 430 is represented by L3′ and the length in the axial direction of the first internal thread portion 422 that is adjacent to the second non-threaded portion 430 in the axial direction is represented by L4′, it is desirable to set the respective lengths in the axial direction to satisfy L3′<L4′.

When the lengths in the axial direction of the individual portions are set as described above, while the external thread portion 320 and the internal thread portion 420 begin to be screwed together and then completely tightened, the first seal portion 310 and the first non-threaded portion 410 (and the second seal portion 330 and the second non-threaded portion 430) engage with each other, whereby a sealing structure can be formed.

According to such oil well pipes 300 and 400, when the oil well pipe 300 is inserted into the oil well pipe 400 and rotated, the individual external thread portions 322 to 326 are screwed into the individual internal thread portions 422 to 426. In addition, the first seal portion 310 is pressed and fitted into the first non-threaded portion 410, and the annular projecting portions 312 come into close contact with the inner circumferential surface of the first non-threaded portion 410. In addition, the second seal portion 330 is pressed and fitted into the second non-threaded portion 430, and the annular projecting portions 332 come into close contact with the inner circumferential surface of the second non-threaded portion 430.

In addition, it is also possible to configure the oil well pipes 300 and 400 so that the tip portion 300 a of the oil well pipe 300 engages with or comes into contact with the receiving portion 400 a of the oil well pipe 400, and, in such a case, the individual external thread portions 322 to 326 and the individual internal thread portions 422 to 426 being tightened to each other more than necessary are inhibited.

In addition, since the annular projecting portions 312 come into close contact with the first non-threaded portion 410, the annular projecting portions 332 come into close contact with the second non-threaded portion 430, respectively, and the annular projecting portions 312 and 332 are pressed to come into contact with the first non-threaded portion 410 or the second non-threaded portion 430, it is possible to form a seal structure capable of coping with situations where the relative dislocation between the oil well pipe 300 and the oil well pipe 400 is accompanied by the input of pulling, contracting, or the like of the oil well pipes 300 and 400 and to form a seal structure that is capable of following the deformation such as bending of the oil well pipes 300 and 400 and has high sealing performance.

Furthermore, a sealing structure may be provided to the annular recessed portion 314. Specifically, a sealing structure can be configured by storing a solid fat that becomes soft and expands at a predetermined temperature or higher (described in detail below) in the annular recessed portion 314 and fitting and/or burying a resin ring made of polyethylene, polypropylene, or the like into the annular recessed portion 314. This sealing structure is capable of further improving the sealing performance.

In addition, as described above, in the first external thread portion 322, since the shape or the like of the threaded region is set so that the total cross-sectional area of the entire thread region becomes equal to or larger than the dimension obtained by multiplying the circular region A area by root 3, the first external thread portion 322 has a higher shear strength than the tensile strength of a bare pipe corresponding to the area of the circular region A. Similarly, the second external thread portion 324 has a higher shear strength than the tensile strength of a bare pipe corresponding to the area of the circular region B, and the third external thread portion 326 has a higher shear strength than the tensile strength of a bare pipe corresponding to the area of the circular region C.

Therefore, the entire external thread portion 320 has a higher shear strength than the tensile strength in the region where the external thread portion 320 is not formed in the oil well pipe 300, and it is possible to prevent the shear failure of the external thread portion 320 from occurring before the occurrence of the axial failure of the oil well pipe 300 at the time of pulling both oil well pipes 300 and 400 to separate the oil well pipes 300 and 400 from each other along the axial direction in a state where the external thread portion 320 has been screwed to the internal thread portion 420.

In the same manner as described above, in the first internal thread portion 422, since the shape or the like of the threaded region is set so that the total cross-sectional area of the entire thread region becomes equal to or larger than the dimension obtained by multiplying the circular region D area by root 3, the first internal thread portion 422 has a higher shear strength than the tensile strength of a bare pipe corresponding to the area of the circular region D. Similarly, the second internal thread portion 424 has a higher shear strength than the tensile strength of a bare pipe corresponding to the area of the circular region E, and the third internal thread portion 426 has a higher shear strength than the tensile strength of a bare pipe corresponding to the area of the circular region F.

Therefore, the entire internal thread portion 420 has a higher shear strength than the tensile strength in the region where the internal thread portion 420 is not formed in the oil well pipe 400, and it is possible to prevent the shear failure of the internal thread portion 420 from occurring before the occurrence of the axial failure of the oil well pipe 400 at the time of pulling both oil well pipes 300 and 400 to separate the oil well pipes 300 and 400 from each other along the axial direction in a state where the internal thread portion 420 has been screwed to the external thread portion 320.

As described above, the shear strengths of the external thread portion 320 and the internal thread portion 420 are higher than the tensile strengths of the respective regions where the thread portion is not formed in the oil well pipes 300 and 400, and it is possible to reliably cause the axial failure of any of the oil well pipes 300 and 400 before the shearing of the thread at the time of applying a tensile force to separate the oil well pipes 300 and 400 from each other in a state where the oil well pipes 300 and 400 have been connected to each other.

The tensile strength of each of the first external thread portion 322, the second external thread portion 324, the third external thread portion 326, the first internal thread portion 422, the second internal thread portion 424, and the third internal thread portion 426 becomes the lowest tensile strength among the tensile strengths of the total shear area a at the time of continuously shearing the substantially effective diameter portion in a fitting region where intended external thread portions or internal thread portions are screwed to each other in a spiral shape along the thread, the cross-sectional area b of a transverse section in the effective diameter portion of an intended external thread portion and the cross-sectional area c of a transverse section in the effective diameter portion of an intended internal thread portion. Here, the total shear area and the cross-sectional areas can be set to satisfy a relationship of a≥√3·b^a≥√3·c, and in this case, the tensile strength of the intended external thread portion or internal thread portion can be calculated as a value obtained by multiplying b and/or c by the tensile strength of a material per unit area.

In addition, the first external thread portion and the third external thread portion are disposed before and after the second external thread portion in the screwing advancement direction, and the first internal thread portion and the third internal thread portion are disposed before and after the second internal thread portion in the screwing advancement direction, whereby the external thread portion and the internal thread portion are screwed together throughout the entire spiral thread region. Therefore, a non-engagement portion of the thread, which has been present in a joint that is referred to as a so-called flash joint of the related art, is formed on neither the tip side nor the base end side of a tapered thread, a decrease in the strength due to the presence of the non-engagement portion is prevented, and consequently, it is possible to strongly connect oil well pipes.

In addition, according to the oil well pipes 300 and 400 having the above-described structure, it is possible to increase the strength of a connection portion when the external thread portion 320 and the internal thread portion 420 are screwed and tightened together. Here, in the evaluation of the tensile strength, it is possible to consider a criterion of the yield rate of the connection portion that is the rate of the tensile strength of the connection portion with respect to the yield strength of the base pipe portion of the oil well pipe 300 or 400. The yield rate of the connection portion can be expressed as {failure strength of connection portion}/{total cross-sectional yield strength of bare pipe portion}.

For example, in a case where a steel material having a strength chart equivalent to Class 10.9 in the bolt strength chart is used, the cross-sectional area of the connection portion is represented by S₁ (mm²), and the cross-sectional area of the bare pipe portion is represented by S₀ (mm²), the yield rate can be expressed as Formula (1).

$\begin{matrix} \left\lbrack {{Formula}\mspace{14mu}(1)} \right\rbrack & \; \\ {\mspace{79mu}{{{yield}\mspace{14mu}{rate}} = \frac{\left\{ {S_{1}\mspace{14mu}\left( {mm}^{2} \right) \times 1040\mspace{14mu}\left( {N\text{/}{mm}^{2}} \right)} \right\}}{\left\{ {S_{0}\mspace{14mu}\left( {mm}^{2} \right) \times 936\mspace{14mu}\left( {N\text{/}{mm}^{2}} \right)} \right\}}}} & (1) \end{matrix}$

Here, the former numeric value ‘10’ in the strength chart is the tensile strength and indicates a numeric value set to at least approximately 1/100 of 1040 (N/mm²). The latter numeric value ‘9’ is the yield strength and guarantees that the rate with respect to the tensile strength of 1040 (N/mm²) is 90 percent. That is, the yield strength is 1040×0.9=936 (N/mm²).

The cross-sectional area S₁ is determined by the cross-sectional area of the external thread portion 320 or the cross-sectional area of the internal thread portion 420. That is, in the external thread portion 320, the range from the inner diameter of the oil well pipe 300 shown in FIG. 13A and FIG. 13B to the effective diameter of the first external thread portion 322 affects the dimension of the cross-sectional area. In the internal thread portion 420, the range from the outer form of the oil well pipe 400 shown in FIG. 15A and FIG. 15B to the effective diameter of the third internal thread portion 426 affects the dimension of the cross-sectional area.

Therefore, the external thread portion 320 is set so that the first external thread portion 322 is radially expanded, the internal thread portion 420 is set so that the third internal thread portion 426 is radially contracted, and, for example, in a case where the cross-sectional area S₁ is set to become as large as approximately 80% of the cross-sectional area S₀, the cross-sectional area S₁ in Formula 1 can be expressed as the cross-sectional area S₀×0.8.

Formula (2) obtained by assigning individual values into Formula 1 will be shown.

$\begin{matrix} \left\lbrack {{Formula}\mspace{14mu}(2)} \right\rbrack & \; \\ {\mspace{79mu}{{{yield}\mspace{14mu}{rate}} = {\frac{\left\{ {S_{0}\mspace{14mu}\left( {mm}^{2} \right) \times 0.8 \times 1040\mspace{14mu}\left( {N\text{/}{mm}^{2}} \right)} \right\}}{\left\{ {S_{0}\mspace{14mu}\left( {mm}^{2} \right) \times 936\mspace{14mu}\left( {N\text{/}{mm}^{2}} \right)} \right\}} = {\frac{\left\{ {S_{0}\mspace{14mu}\left( {mm}^{2} \right) \times 832\mspace{14mu}\left( {N\text{/}{mm}^{2}} \right)} \right\}}{\left\{ {S_{0}\mspace{14mu}\left( {mm}^{2} \right) \times 936\mspace{14mu}\left( {N\text{/}{mm}^{2}} \right)} \right\}} = 0.89}}}} & (2) \end{matrix}$

As shown in Formula (2), it is possible to obtain a yield rate of approximately 89 percent.

In addition, as the rate of the yield strength with respect to the tensile strength of a material to be selected decreases, a higher yield rate can be obtained. For example, in a case where a material having a rate of the yield strength with respect to the tensile strength of 70 percent is selected and the cross-sectional area S₁ is set to be as large as approximately 80% of the cross-sectional area S₀, a yield rate of approximately 114 percent can be obtained.

In addition, even in a case where a material having a high rate of the yield strength with respect to the tensile strength has been selected, as the cross-sectional area S₁ is made closer to the cross-sectional area S₀, it becomes possible to obtain the oil well pipes 300 and 400 having a higher yield rate. It is needless to say that, realistically, due to a problem with processing accuracy, a problem with the strength in the thinnest portion of the first internal thread portion 422, the second non-threaded portion 430, the third external thread portion 326, or the first seal portion 310, or the like, there is a need to consider that it is not preferable to reduce the thickness to a certain thickness or thinner.

In addition, for example, in an oil well pipe manufactured using a material having a tensile strength of 965 (N/mm²) and a yield strength of 862 (N/mm²) (a material having a rate of the yield strength with respect to the tensile strength of approximately 89 percent), in a case where the bare pipe portion is 400 mm in outer diameter and 19 mm in thickness and the thinnest portion of the first internal thread portion 422, the second non-threaded portion 430, the third external thread portion 326, or the first seal portion 310 is set to 1.4 mm, according to Formula 1, a yield rate of approximately 103 percent can be obtained. In the related art, the yield rate of an oil well pipe that is manufactured using the same material is approximately 70%. This fact shows that, when the external thread portion 320 and the internal thread portion 420 according to the present disclosure are screwed together, that is, the straight threads are screwed together, and the tapered threads are screwed together, it becomes possible to set the thickness to be thinner, and it is possible to obtain an extremely high yield rate compared with oil well pipes of the related art.

The thread shapes in the external thread portion 320 may be changed in each of the first external thread portion 322, the second external thread portion 324, and the third external thread portion 326 or may be the same shape in all of the portions. For example, the thread may have a saw blade shape in each of the external thread portions 322 to 326, and it is possible to provide a symmetric shape such as a triangular thread or a trapezoidal thread in the first external thread portion 322 and the third external thread portion 326 both having a straight thread and to provide a saw blade shape in the second external thread portion 324 having a tapered thread. Since the internal thread portion 420 is screwed to the external thread portion 320, it is preferable that the thread in each of the internal thread portions 422 to 426 are corresponding to the thread in each of the external thread portions 322 to 326, and basically, the pitches are set to be equal to each other.

Here, FIG. 20 is a view showing the tensile strengths and the compressive strengths of test bodies of oil well pipe coupling structures in which the oil well pipe 300 and the oil well pipe 400 that correspond to each of thread shapes are screwed and coupled together. The failure strength of the oil well pipe coupling structure is affected by the thread shape in the external thread portion 320 and the form of the thread in the internal thread portion 420, which corresponds to the thread shape in the external thread portion 320.

Specifically, the oil well pipe 300 has the circular region A, the circular region B, and the circular region C, which have been described above, the oil well pipe 400 has the circular region D, the circular region E, and the circular region F, which have been described above, and these regions are screwed and coupled together to form the test body. Furthermore, the area of the circular region A is set to equal to or smaller than a third of the total cross-sectional area of a transverse section of the oil well pipe 300, and the thread engagement effective length, the number of the thread, the pitches, and the like are set so that the total shear area of the thread in the first external thread portion 322 becomes equal to or larger than double root 3 of the circular region A area.

In addition, the area of the circular region B is set to equal to or larger than a third of the total cross-sectional area of a transverse section of the oil well pipe 300, and the thread engagement effective length, the number of the thread, the pitches, and the like are set so that the total shear area of the thread in the second external thread portion 324 becomes equal to or larger than double root 3 of the circular region B area.

In addition, the area of the circular region C is set to equal to or smaller than a third of the total cross-sectional area of a transverse section of the oil well pipe 300, and the thread engagement effective length, the number of the thread, the pitches, and the like are set so that the total shear area of the thread in the third external thread portion 326 becomes equal to or larger than double of the circular region C area.

In addition, the area of the circular region D is set to a third or less of the total cross-sectional area of a transverse section of the oil well pipe 400. In addition, the area of the circular region E is set to a third or more of the total cross-sectional area of the transverse section of the oil well pipe 400. In addition, the area of the circular region F is set to a third or less of the total cross-sectional area of the transverse section of the oil well pipe 400. The circular region D is configured to have an area substantially corresponding to the circular region A, the circular region E is configured to have an area substantially corresponding to the circular region B, and the circular region F is configured to have an area substantially corresponding to the circular region C.

The results of tensile tests and compressive tests carried out on the oil well pipe 300 and the oil well pipe 400 that have the above-described configuration after preparing two types of test bodies and setting mutually different thread shapes are shown in FIG. 20. The thread engagement lengths, the numbers of the thread, the pitches, and the like of the respective test bodies were set to be equal to each other.

Here, regarding the thread shapes, one was a saw blade shape in which the thread angle was set to 60°, and the other was a triangular thread having a symmetrical cross-sectional shape in which the thread angle was set to 70°. In FIG. 20, the broken line indicates the result of the tensile test of the oil well pipe coupled body (test body) formed by screwing and coupling the oil well pipes 300 and 400 each having a thread angle of 70° to each other. The alternate long and short dashed line indicates the result of the tensile test of the oil well pipe coupled body (test body) having a saw blade shape with a thread angle of 60°. The solid line indicates the result of the compressive test of the oil well pipe coupled body (test body) having a saw blade shape with a thread angle of 60°. The alternate long and two short dashed line indicates the result of the compressive test of the oil well pipe coupled body (test body) having a thread angle of 70°.

In the tensile test, the oil well pipe 300 and the oil well pipe 400 that formed the oil well pipe coupled body (test body) were mounted in a tester through jigs, respectively, and furthermore, the oil well pipe coupled body were pulled in a state where the external thread portion 320 and the internal thread portion 420 were screwed together until axial failure occurred.

In addition, in the compressive test, in a state where the oil well pipe 300 and the oil well pipe 400 that formed the oil well pipe coupled body (test body) were screwed together in advance, the oil well pipes were mounted in a test through jigs and compressed until the oil well pipes broke.

In the case of comparing the broken line and the solid line in FIG. 20, that is, the compressive test results, it was found that the compressive strength was higher in the oil well pipe coupled body (test body) having the thread with a thread angle of 70° and a symmetrical cross-sectional shape than in the oil well pipe coupled body (test body) having the thread with a saw blade shape and a thread angle of 60°.

On the other hand, in the case of comparing the alternate long and short dashed line and the alternate long and two short dashed line in FIG. 20, that is, the tensile test results, it was found that the tensile strength was higher in the oil well pipe coupled body (test body) having the thread with a saw blade shape and a thread angle of 60° than in the oil well pipe coupled body (test body) having the thread with a thread angle of 70°. Based on such compressive test results and tensile test results, it can be said that the thread shape that is strong against compression and the thread shape that is strong against pulling are different from each other.

Therefore, it is desirable to appropriately set the thread shape depending on where to use the oil well pipe coupled body (test body), and, for example, the oil well pipe coupled body (test body) to be disposed in an environment that is susceptible to tensile stress is preferably an oil well pipe coupled body having a thread with a saw blade shape. In addition, the oil well pipe coupled body (test body) to be disposed in an environment that is susceptible to compressive stress is preferably an oil well pipe coupled body having thread with a thread angle of 70° and a symmetrical cross-sectional shape. Alternatively, in the case of being disposed in an environment or a use that is susceptible to both tensile stress and compressive stress, an oil well pipe coupled body having a thread with a thread angle of 70° and a symmetrical cross-sectional shape is preferably selected rather than an oil well pipe coupled body having a thread with a symmetrical cross-sectional shape.

In addition, the circular projecting portions 312 and 332 and the circular recessed portions 314 and 334 are provided in the outer circumferences of the individual seal portions 310 and 330, circular projecting portions and circular recessed portions may be provided on the oil well pipe 400 side. That is, circular projecting portions and circular recessed portions may be formed on the inner circumferential surfaces of the individual non-threaded portions 410 and 430, and the circular projecting portions may be brought into close contact with the outer circumferential surfaces of the individual seal portions 310 and 330. In this case, it is possible to provide a circumferential surface shape or a substantially conical circumferential surface shape to the outer circumferences of the individual seal portions.

In addition, the first seal portion 310 and the second seal portion 330 are provided on the tip side and the base end side of the oil well pipe 300, but the seal portion may be provided on any one of the tip side or the base end side alone. That is, the seal portion 310 may be provided only on the tip side of the oil well pipe 300 as shown in FIG. 16A or the seal portion 330 may be provided only on the base end side of the external thread portion 320 in the oil well pipe 300 as shown in FIG. 16B. In such a case, in the oil well pipe 400, the non-threaded portion may be formed only in a place that corresponds to the seal portion. That is, the non-threaded portion may be formed on any one of one end side or the other end side of the internal thread portion alone.

In addition, the shape of the tip portion 300 a that regulates the insertion of the oil well pipe 300 into the oil well pipe 400 can be appropriately set. For example, the tip portion 300 a may be a tip portion inclined at a shallow angle of, for example, less than 45° with respect to a direction perpendicular to the axis as shown in FIG. 17A or, additionally, a tip portion formed in a curved surface shape as shown in FIG. 17B, that is, a tip portion formed by roundly chamfering the tip portion of the oil well pipe 300. In addition, the end surface of the oil well pipe 300 in which the corner portions each have a square shape as shown in FIG. 17C may be used as the tip portion 300 a. When the tip portion 300 a is an inclined surface or has a curved surface shape, a load may concentrate on the receiving portion 400 a through the inclined tip portion 300 a outward in the radial direction, which creates a concern that the oil well pipe 400 may radially expand. Therefore, when the tip portion 300 a is configured to come into surface-contact with the receiving portion 400 a in a broad range including the end surface of the oil well pipe 300, the load is dispersed as much, and it is possible to inhibit the load exerted outward in the radial direction from acting on the receiving portion 400 a.

It is needless to say that the shape of the base end portion 300 b that is positioned on the base end side of the external thread portion 320 of the oil well pipe 300 can also be appropriately set in the same manner. In addition, the shape of the receiving portion 400 a can be set so as to correspond to the shape of the tip portion 300 a or the base end portion 300 b.

In addition, the insertion of the oil well pipe 300 into the oil well pipe 400 may be regulated with means other than the provision of the tip portion 300 a or the base end portion 300 b. For example, the pitches of the terminals of the thread in any one or both external thread portion 320 and internal thread portion 420 are set to be short. That is, the pitches of at least one of some of the thread on the base end side of the external thread portion 320 and some of the thread on the inner side of the internal thread portion 420 in the insertion direction are set to be short. In such a case, it is possible to provide a braking structure that regulates screwing advancement at the place where the pitches are set to be short when the external thread portion 320 and the internal thread portion 420 are screwed together. It is needless to say that, in this case, there is no need to bring the tip portion 300 a and the receiving portion 400 a into contact with each other; however, conversely, the tip portion 300 a and the receiving portion 400 a may be brought into contact with each other to disperse the load between the braking structure and the tip portion 300 a or the receiving portion 400 a.

In addition, the outer diameter of the external thread portion 320 and/or the inner diameter of the internal thread portion 420 may be set in order to improve the sealing property between the external thread portion 320 and the internal thread portion 420. That is, the second external thread portion 324 with the tapered thread in the external thread portion 320 presses the second internal thread portion 424 with the tapered thread outward from the inside to elastically deform and/or plastically deform at least one of the second external thread portion 324 and the second internal thread portion 424.

For example, the maximum outer diameter (or the maximum effective diameter) of the second external thread portion 324 is set to be larger than the maximum inner diameter (or the maximum effective diameter) of the second internal thread portion 424 and/or the minimum outer diameter (or the minimum effective diameter) of the second external thread portion 324 is set to be larger than the minimum inner diameter (or the minimum effective diameter) of the second internal thread portion 424 so that the internal thread portion 420 is capable of slightly expanding outward in the radial direction in a state where the external thread portion 320 and the internal thread portion 420 are fully screwed together. In such a case, the second external thread portion 324 and the second internal thread portion 424 are capable of interfering each other in the radial direction at least in a part, and, at the time of screwing the external thread portion 320 and the internal thread portion 420, the internal thread portion 420 radially expands while elastically deforming or plastically deforming, consequently, the external thread portion 320 and the internal thread portion 420 come into close contact with each other, and thereby the sealing property improves.

It is needless to say that the dimension of the inner diameter of the second internal thread portion 424 with respect to the outer diameter of the second external thread portion 324 may be set so that the second external thread portion 324 elastically deforms and/or plastically deforms inward in the radial direction. In addition, in the above description, the outer diameter of the second external thread portion 14 and the inner diameter of the second internal thread portion 24 are set; however, additionally, the thicknesses of the pipes in the second external thread portion 324 and/or the second internal thread portion 424 may be set.

In a case where the second external thread portion 324 and the second internal thread portion 424 are configured to interfere with each other, a first interference mitigation portion 323 may be provided between the first external thread portion 322 and the second external thread portion 324 (refer to FIG. 24A), and a second interference mitigation portion 425 may be provided between the second internal thread portion 424 and the third internal thread portion 426 (refer to FIG. 24B).

The first interference mitigation portion is disposed in an outwardly projecting (projecting outward in the radial direction) boundary region between the first external thread portion 322 and the second external thread portion 324 and is configured by forming the first interference mitigation portion in a curved surface shape, an inclined surface shape, or the like or forming a non-threaded region so as not to interfere with the second internal thread portion 424 in the axial direction. In addition, the second interference mitigation portion is disposed in an outwardly projecting (projecting inward in the radial direction) boundary region between the second internal thread portion 424 and the third internal thread portion 426 and is configured by forming the first interference mitigation portion in a curved surface shape, an inclined surface shape, or the like or forming a non-threaded region so as not to interfere with the third external thread portion 324 in the axial direction.

The provision of both interference mitigation portions makes it possible to avoid interference between the first external thread portion 322 and the second internal thread portion 424 and to avoid interference between the third internal thread portion 426 and the second external thread portion 324.

In addition, the first interference mitigation portion and/or the second interference mitigation portion may have a surface shape that mitigates or avoids interference with the second internal thread portion 424 (or the second external thread portion 324), but the first interference mitigation portion may include the thread that can be screwed to the second internal thread portion 424, and similarly, the second interference mitigation portion may include the thread that can be screwed to the second external thread portion 324.

In addition, as the solid fat, for example, there can be a thermoplastic resin, a glass fiber reinforced resin into which the thermoplastic resin is mixed, a thermoplastic elastomer, or the like. As the thermoplastic resin, for example, general-purpose plastic (polyethylene, high-density polyethylene, medium-density polyethylene, low-density polyethylene, polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene, polyvinyl acetate, polyurethane, polytetrafluoroethylene, an acrylonitrile butadiene styrene resin, an AS resin, an acrylic resin, or the like), engineering plastic (polyamide, nylon, polyacetal, polycarbonate, modified polyphenylene ether, polyethylene terephthalate, glass fiber reinforced polyethylene terephthalate, polybutylene terephthalate, cyclic polyolefin, or the like), super engineering plastic (polyphenylensulfide, polytetrafluoroethylene, polysulfone, polyether sulfone, amorphous polyarylate, a liquid crystal polymer, polyether ether ketone, thermoplastic polyimide, polyamide-imide, or the like), or the like is applicable and is selected depending on the application temperature, the thermal expansion coefficient, required chemical resistance, or the like.

In addition, in the external thread portion 320 of the oil well pipe 300, the lengths of the regions along the axial direction of the portion with the straight thread and of the portion with the tapered thread may be appropriately set. For example, the region of the second external thread portion 324 along the axial direction may be set to be long compared with the first external thread portion 322 and/or the third external thread portion 326. That is, the portion with the tapered thread may be set to be longer in the axial direction than the portion with the straight thread as shown in FIG. 19A. In addition, the length of the region along the axial direction of the second external thread portion 324 may be set to be shorter than the first external thread portion 322 and/or the third external thread portion 326 as shown in FIG. 19B, that is, the regions along the axial direction of the first external thread portion 322 and/or the third external thread portion 326 may be set to be long compared with the second external thread portion 324. It is needless to say that, for the oil well pipe 400 having the internal thread portion 420 that is screwed to the external thread portion 320 as well, it is possible to appropriately set the lengths of the regions along the axial direction of the portion with the straight thread and of the portion with the tapered thread in the same manner as for the oil well pipe 300.

In a case where the region along the axial direction of the first external thread portion 322 or the third external thread portion 326 has been set to be long, it is possible to reliably use the tensile failure portion as the base portion of the external thread portion 320 or as the base portion of the internal thread portion 420 even when the thickness of the residual thickness portion of the first internal thread portion 422 or the third internal thread portion 426 (the thickness portion of the pipe remaining after the thread portion is removed in the first internal thread portion 422 or the third internal thread portion 426) that is screwed to the first external thread portion 322 or the third external thread portion 326. 

1. An oil well pipe coupling structure comprising: a first oil well pipe having an external thread portion on an outer circumferential surface; and a second oil well pipe that has an internal thread portion that is screwed to the external thread portion on an inner circumferential surface and can be connected to the first oil well pipe, wherein the external thread portion is made up of a first external thread portion having a diameter of a predetermined dimension and a substantially tapered second external thread portion that is disposed on one end portion side with respect to the first external thread portion and gradually contracts radially toward one end side, the internal thread portion is made up of a first internal thread portion that is disposed on an opening end side and has a diameter of a predetermined dimension and a substantially tapered second internal thread portion that gradually contracts radially from the first internal thread portion side toward an inner side, the first external thread portion is screwed to the first internal thread portion, the second external thread portion is screwed to the second internal thread portion, and the first oil well pipe and the second oil well pipe are coupled together.
 2. The oil well pipe coupling structure according to claim 1, wherein a diameter of the second external thread portion is equal to or smaller than the diameter of the first external thread portion, and a diameter of the second internal thread portion is equal to or larger than the diameter of the first internal thread portion.
 3. The oil well pipe coupling structure according to claim 1, wherein a thread shape in the first external thread portion and/or the second external thread portion forms a substantially saw blade shape, and the thread shape in the first internal thread portion and/or the second internal thread portion forms a substantially saw blade shape.
 4. The oil well pipe coupling structure according to claim 3, wherein, in the thread shape of the first external thread portion and/or the second external thread portion, a flank angle of a flank surface that receives a pressure when the first oil well pipe and the second oil well pipe are pulled to be separated from each other along an axial direction from a coupled state of the first oil well pipe and the second oil well pipe is equal to or smaller than a right angle with respect to an axial core of the first oil well pipe.
 5. The oil well pipe coupling structure according to claim 3, wherein, in the thread shape of the first internal thread portion and/or the second internal thread portion, a flank angle of a flank surface that receives a pressure when the first oil well pipe and the second oil well pipe are pulled to be separated from each other along an axial direction from a coupled state of the first oil well pipe and the second oil well pipe is equal to or smaller than a right angle with respect to an axial core of the second oil well pipe.
 6. The oil well pipe coupling structure according to claim 1, wherein the first external thread portion and the second external thread portion are equal to each other in pitches of thread.
 7. The oil well pipe coupling structure according to claim 1, wherein the first internal thread portion and the second internal thread portion are equal to each other in pitches of thread.
 8. The oil well pipe coupling structure according to claim 1, wherein pitches of thread in the external thread portion and pitches of thread in the internal thread portion are equal to each other.
 9. The oil well pipe coupling structure according to claim 1, wherein, in the external thread portion, pitches of thread gradually decrease from the first external thread portion side toward the second external thread portion side, and in the internal thread portion, pitches of thread gradually decrease from the first internal thread portion side toward the second internal thread portion side.
 10. The oil well pipe coupling structure according to claim 1, wherein an outer diameter of a non-engagement region in the first oil well pipe with respect to the second oil well pipe is substantially equal to an outer diameter of the second oil well pipe.
 11. The oil well pipe coupling structure according to claim 1, wherein an inner diameter of a hollow in the first oil well pipe is substantially equal to an inner diameter of a non-engagement region of the second oil well pipe with respect to the first oil well pipe.
 12. The oil well pipe coupling structure according to claim 1, wherein a region along an axial direction is longer in the second external thread portion than in the first external thread portion.
 13. The oil well pipe coupling structure according to claim 1, wherein a region along an axial direction is longer in the second internal thread portion than in the first internal thread portion. 14.-56. (canceled)
 57. The oil well pipe coupling structure according to claim 1, wherein the external thread portion has a third external thread portion that is disposed on the one end side with respect to the second external thread portion and has a diameter of a predetermined dimension, the internal thread portion has a third internal thread portion that is disposed on the inner side with respect to the second internal thread portion and has a diameter of a predetermined dimension, and the third external thread portion can be screwed to the third internal thread portion.
 58. The oil well pipe coupling structure according to claim 57, wherein the third external thread portion has an effective diameter that is equal to or larger than an effective diameter of the second external thread portion and has a straight thread shape, and the third internal thread portion has an effective diameter that is equal to or larger than an effective diameter of the second internal thread portion and has a straight thread shape.
 59. The oil well pipe coupling structure according to claim 57, wherein a region along an axial direction is longer in the second external thread portion than in the first external thread portion and/or the third external thread portion.
 60. The oil well pipe coupling structure according to claim 57, wherein a region along an axial direction is shorter in the second external thread portion than in the first external thread portion and/or the third external thread portion.
 61. The oil well pipe coupling structure according to claim 57, wherein a region along an axial direction is longer in the second internal thread portion than in the first internal thread portion and/or the third internal thread portion.
 62. The oil well pipe coupling structure according to claim 57, wherein a region along an axial direction is shorter in the second internal thread portion than in the first internal thread portion and/or the third internal thread portion.
 63. The oil well pipe coupling structure according to claim 57, wherein the external thread portion has a first circular region including the first external thread portion, a second circular region including the second external thread portion, and a third circular region including the third external thread portion that are virtually divided in a radial direction, an area of the first circular region is equal to or smaller than a third of a total cross-sectional area of a transverse section of the first oil well pipe, an area of the second circular region is equal to or larger than a third of the total cross-sectional area of the transverse section of the first oil well pipe, an area of the third circular region is equal to or smaller than a third of the total cross-sectional area of the transverse section of the first oil well pipe, the internal thread portion has a fourth circular region including the first internal thread portion, a fifth circular region including the second internal thread portion, and a sixth circular region including the third internal thread portion that are virtually divided in a radial direction, an area of the fourth circular region is equal to or smaller than a third of a total cross-sectional area of a transverse section of the second oil well pipe, an area of the fifth circular region is equal to or larger than a third of the total cross-sectional area of the transverse section of the second oil well pipe, and an area of the sixth circular region is equal to or smaller than a third of the total cross-sectional area of the transverse section of the second oil well pipe.
 64. The oil well pipe coupling structure according to claim 57, wherein the first external thread portion and/or the third external thread portion have a thread with a symmetrical shape, the first internal thread portion has the thread shape corresponding to the thread of the first external thread portion, and the third internal thread portion has the thread shape corresponding to the thread of the third external thread portion.
 65. The oil well pipe coupling structure according to claim 1, wherein a sealing structure is provided between an outer circumferential surface of a tip portion of the first oil well pipe and an inner circumferential surface on a base end side of the internal thread portion of the second oil well pipe and/or between an outer circumferential surface on a base end side of the external thread portion of the first oil well pipe and an inner circumferential surface of the tip portion of the second oil well pipe.
 66. The oil well pipe coupling structure according to claim 57, wherein a sealing structure is provided between an outer circumferential surface of a tip portion of the first oil well pipe and an inner circumferential surface on an inner side of the internal thread portion of the second oil well pipe and/or between an outer circumferential surface on a base end side of the external thread portion of the first oil well pipe and an inner circumferential surface of the opening portion of the second oil well pipe, and in the sealing structure, a length in an axial direction of the outer circumferential surface of a tip portion of the first oil well pipe is set to shorter than a length in the axial direction of the third internal thread portion of the second oil well pipe and/or a length in the axial direction of the outer circumferential surface on the base end side of the external thread portion of the first oil well pipe is set to shorter than a length in the axial direction of the inner circumferential surface on the opening side of the second oil well pipe.
 67. An oil well pipe having a hollow structure, comprising: an external thread portion on an outer circumferential surface, wherein the external thread portion is made up of a first external thread portion having a diameter of a predetermined dimension and a substantially tapered second external thread portion that is disposed on one end portion side with respect to the first external thread portion and gradually contracts radially toward one end side, and the external thread portion is screwed to a different member having an internal thread portion.
 68. The oil well pipe according to claim 67, wherein a diameter of the second external thread portion is equal to or smaller than the diameter of the first external thread portion.
 69. The oil well pipe according to claim 67, wherein the thread shape in the first external thread portion and/or in the second external thread portion forms a substantially saw blade shape.
 70. The oil well pipe according to claim 69, wherein, in the thread shape of the first external thread portion and/or the second external thread portion, a flank angle of a flank surface that receives a pressure when the oil well pipe is pulled to be separated in an axial direction from a state where the oil well pipe is screwed to the different member is equal to or larger than a right angle with respect to an axial core of the oil well pipe.
 71. The oil well pipe according to claim 67, wherein a region along an axial direction is longer in the second external thread portion than in the first external thread portion.
 72. The oil well pipe according to claim 67, wherein the external thread portion has a third external thread portion that is disposed on the one end side with respect to the second external thread portion and has a diameter of a predetermined dimension.
 73. The oil well pipe according to claim 72, wherein the third external thread portion has an effective diameter that is equal to or larger than an effective diameter of the second external thread portion and has a straight thread shape.
 74. The oil well pipe according to claim 72, wherein a region along an axial direction is longer in the second external thread portion than in the first external thread portion and/or the third external thread portion.
 75. The oil well pipe according to claim 72, wherein a region along an axial direction is shorter in the second external thread portion than in the first external thread portion and/or the third external thread portion.
 76. The oil well pipe according to claim 72, wherein the external thread portion has a first circular region including the first external thread portion, a second circular region including the second external thread portion, and a third circular region including the third external thread portion that are virtually divided in a radial direction, an area of the first circular region is equal to or smaller than a third of a total cross-sectional area of a transverse section of the oil well pipe, an area of the second circular region is equal to or larger than a third of the total cross-sectional area of the transverse section of the oil well pipe, and an area of the third circular region is equal to or smaller than a third of the total cross-sectional area of the transverse section of the oil well pipe.
 77. The oil well pipe according to claim 72, wherein the first external thread portion and the third external thread portion have the thread having a symmetrical shape.
 78. The oil well pipe according to claim 67, wherein, on a tip side and/or a base end side of the external thread portion, an outer circumferential surface that can be brought into close contact with an inner circumferential surface of the different member substantially throughout an entire circumference when the external thread portion has been fitted into the internal thread portion of the different member is provided.
 79. The oil well pipe according to claim 72, wherein, on a tip side and/or a base end side of the external thread portion, an outer circumferential surface that can be brought into close contact with an inner circumferential surface of the different member substantially throughout an entire circumference when the external thread portion has been fitted into the internal thread portion of the different member is provided, and a length in an axial direction of the outer circumferential surface on the tip side of the external thread portion is set to shorter than a length in the axial direction of the third external thread portion and/or a length in the axial direction of the outer circumferential surface on the base end side of the external thread portion is set to shorter than a length in the axial direction of the first external thread portion.
 80. An oil well pipe having a hollow structure, comprising: an internal thread portion on an inner circumferential surface, wherein the internal thread portion is made up of a first internal thread portion having a diameter of a predetermined dimension on an opening end side and a substantially tapered second internal thread portion that gradually expands radially along a direction away from the first internal thread portion, and the internal thread portion is screwed to a different member having an external thread portion.
 81. The oil well pipe according to claim 80, wherein a diameter of the second internal thread portion is equal to or larger than the diameter of the first internal thread portion.
 82. The oil well pipe according to claim 80, wherein the thread shape in the first internal thread portion and/or the second internal thread portion forms a substantially saw blade shape.
 83. The oil well pipe according to claim 82, wherein, in the thread shape of the first internal thread portion and/or the second internal thread portion, a flank angle of a flank surface that receives a pressure when the oil well pipe is pulled to be separated along an axial direction from a state where the oil well pipe has been screwed to the different member is equal to or larger than a right angle with respect to an axial core of the oil well pipe.
 84. The oil well pipe according to claim 80, wherein a region along an axial direction is longer in the second internal thread portion than in the first internal thread portion.
 85. The oil well pipe according to claim 80, wherein the internal thread portion has a third internal thread portion that is disposed on the opening end side with respect to the second internal thread portion and has a diameter of a predetermined dimension.
 86. The oil well pipe according to claim 85, wherein the third internal thread portion has an effective diameter that is equal to or larger than an effective diameter of the second internal thread portion and has a straight thread shape.
 87. The oil well pipe according to claim 85, wherein a region along an axial direction is longer in the second internal thread portion than in the first internal thread portion and/or the third internal thread portion.
 88. The oil well pipe according to claim 85, wherein a region along an axial direction is shorter in the second internal thread portion than in the first internal thread portion and/or the third internal thread portion.
 89. The oil well pipe according to claim 85, wherein the internal thread portion has a fourth circular region including the first internal thread portion, a fifth circular region including the second internal thread portion, and a sixth circular region including the third internal thread portion that are virtually divided in a radial direction, an area of the fourth circular region is equal to or smaller than a third of a total cross-sectional area of a transverse section of the oil well pipe, an area of the fifth circular region is equal to or larger than a third of the total cross-sectional area of the transverse section of the oil well pipe, and an area of the sixth circular region is equal to or smaller than a third of the total cross-sectional area of the transverse section of the oil well pipe.
 90. The oil well pipe according to claim 85, wherein the first internal thread portion and the third internal thread portion have the thread having a symmetrical shape.
 91. The oil well pipe according to claim 80, wherein, on the opening side and/or an inner side of the internal thread portion, an inner circumferential surface that can be brought into close contact with an outer circumferential surface of the different member substantially throughout an entire circumference when the internal thread portion has been fitted into the external thread portion of the different member is provided.
 92. The oil well pipe according to claim 85, wherein, on the opening side and/or an inner side of the internal thread portion, an inner circumferential surface that can be brought into close contact with an outer circumferential surface of the different member substantially throughout an entire circumference when the internal thread portion has been fitted into the external thread portion of the different member is provided, and a length in an axial direction of the inner circumferential surface on the opening side of the internal thread portion is set to shorter than a length in the axial direction of the third internal thread portion and/or a length in the axial direction of the inner circumferential surface on the inner side of the internal thread portion is set to shorter than a length in the axial direction of the first internal thread portion.
 93. The oil well pipe coupling structure according to claim 1, wherein, when the first oil well pipe and the second oil well pipe are in a state of being coupled to each other, the second external thread portion and the second internal thread portion interfere with each other in a radial direction, and one of the second external thread portion and the second internal thread portion elastically deforms and/or plastically deforms in the radial direction, and the external thread portion has an interference mitigation portion in a boundary region between the first external thread portion and the second external thread portion.
 94. The oil well pipe coupling structure according to claim 57, wherein, when the first oil well pipe and the second oil well pipe are in a state of being coupled to each other, the second external thread portion and the second internal thread portion interfere with each other in a radial direction, and one of the second external thread portion and the second internal thread portion elastically deforms and/or plastically deforms in the radial direction, and the internal thread portion has an interference mitigation portion in a boundary region between the second internal thread portion and the third internal thread portion.
 95. The oil well pipe according to claim 67, wherein the external thread portion has an interference mitigation portion in a boundary region between the first external thread portion and the second external thread portion.
 96. The oil well pipe according to claim 85, wherein the internal thread portion has an interference mitigation portion in a boundary region between the second internal thread portion and the third internal thread portion. 